Download to read offline
![18 GNSS Interference Threats and Countermeasures
of threats. GNSS is particularly prone to unintended and malicious radio-
frequency interference (RFI) due to the extremely low power level of the
signal at the user’s receiver after traveling from the satellite transmitter to the
receiving antenna on the Earth. Due to the weakness of the GNSS signal that
reaches users and a crowded frequency spectrum, GNSS-based services will
be always vulnerable to the presence of interfering signals generated by other
communication systems. A recent example of these risks was the LightSquared
case in the United States, where the GPS receiver operations in the L1 GPS
band have been seriously threatened [1].
Furthermore, GNSS threats include intentional attacks with the objec-
tive of disrupting the target receiver. Recalling that GNSS bandwidths are
protected, the malicious transmission of counterfeit GNSS-like signals, usu-
ally known as spoofing, may become quite dangerous also for civil use of
GNSS. Spoofing and GNSS receiver deception are becoming a threat, as
more applications and infrastructures begin to rely on GNSS position and
time information. Although the vulnerability of GNSS-based civilian infra-
structures is understood, few recognize that severe attacks can be carried out
with self-made spoofing devices composed of a software receiver and trivial
RF front-ends, as recently demonstrated in the United States by researchers
at the University of Austin in Texas [2]. Provision of timing references for
communication networks, agriculture, fishery, and road tolling applications
are just a few examples of markets that would be deeply affected by spoofing
activity designed to elude public authorities or service providers.
Thus, with the growth of civilian GNSS use, unintentional interference,
jamming, and spoofing are emerging security challenges in the civil field.
There are several applications for which it is essential to detect such types of
intentional deception in order to ensure reliable position and time estimations.
The provision of such robustness can protect personal safety or infrastructures
such as power grids, distribution networks, or communication networks for
which GNSS is the provider of timing information. The importance of ensur-
ing a robust receiver with respect to interference and spoofing is crucial for all
types of applications where the concept of security is needed. Hence, evaluat-
ing the possible impact of potential threats on particular services related to
transportation applications (aviation, maritime, railway, road), to emergency
applications oriented to the tracking and tracing of sensitive material (e.g.,
medical or dangerous goods), and to financial/assurance aspects is a priority.
The goal of this book is to provide an overview of the major sources of
interference and spoofing for a GNSS receiver, discussing both the methods
used to assess their impact on the positioning performance as well as the meth-
ods used to protect civilian use of GNSS against unintentional and intentional
6519_Book.indb 18 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-23-2048.jpg)
![20 GNSS Interference Threats and Countermeasures
wave by introducing fluctuations in amplitude and phase; such phenomena
are usually called scintillations [3]. How often GNSS signals are affected by
scintillations depends on solar and geomagnetic activity, geographic location,
season, local time, and signal frequency. Scintillation can be considered a
sort of natural interference interacting with the GNSS signal that causes the
signals to fade and induces a frequency shift in the signal carrier that in some
cases can strongly affect the GNSS receiver. During strong ionospheric events,
amplitude fades and frequency variations can be very challenging for a receiver
and may cause frequent cycle slips and losses of lock of the satellite signals [4].
1.2.2 Multipath
Multipath occurs whenever the user device receives reflected signals in addition
to the direct line-of-sight signal. These replicas of the signals are generated from
the ground, buildings, or trees in terrestrial navigation, whereas signal reflec-
tions from the host-vehicle body are more common in airborne and marine
applications. Multipath can be specular when generated from smooth surfaces
or diffuse when arising from diffuse scatterers and sources of diffraction.
To a certain extent, multipath can then be considered a self-interference,
where the interfering signal is a replica of the signal itself.
1.2.3 Intersystem and Intrasystem Interference
The signal impinging the GNSS receiver antenna at a given frequency is the
combination of the signals broadcast by all the satellites in view. GNSS RF
compatibility addresses the issue of intrasystem (from the same system) and
intersystem (from other systems) interference. Signals belonging to the same
satellite constellation are designed to be theoretically orthogonal (exploiting
code or frequency diversity), and thus they can be separated by the receiver
processing. However, such orthogonality is not perfect and a residual power
is always generating intrasystem interference.
Intersystem interference is due to the fact that several GNSS systems
share the same carriers, and again, some power from the signals of another
system can disrupt the signal of interest. Several methodologies, such as the
effective carrier power to noise density theory introduced in [5], are used during
the design phase of the systems to ensure that a maximum acceptable level of
intersystem interference is respected (see, e.g., [6, 7]). Intra- and intersystem
interference is then a topic that needs to be addressed during the design phase,
and it is beyond the regular users’ capabilities to deal with it.
6519_Book.indb 20 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-25-2048.jpg)
![The Interference Threat21
Due to the growing number of operational satellites in the new GNSS
constellations, the number of satellites that are in view to a user receiver at
the same time is growing as well. This implies increased intersystem interfer-
ence. However, from a user’s standpoint, it can only be reduced by means of
directional antennas that can spatially filter the signals coming from satellites
that are not of interest.
1.2.4 Artificial Interference: Unintentional and Intentional
Interference
The intrinsic power weakness of GNSS signals affects the performance of any
type of receiver, since all the communication systems transmitting at carrier
frequencies close to the band of interest are potential sources of interference
for a GNSS receiver, and even small leakages out of their allocated bandwidth
can be threatening to GNSS signals. Even though unintentional RFI events are
generally unpredictable, their presence has been experienced in the past and
the increasing number of wireless communication infrastructures is increasing
the probability that some power spillover from signal frequencies located near
the GNSS bands could affect the performance of GNSS receivers in a certain
region. The presence of interfering power can be due to several reasons, but
the main effects are caused by harmonics or spurious components generated
by intermodulation products in the communication transmitter.
Jamming refers to intentional transmission of RF energy to hinder a
navigation service by masking GNSS signals with noise. The malicious objec-
tive of jammers is to cause the receiver to lose tracking and to impede signal
reacquisition. Although jamming is a well-known threat in the military appli-
cations, it represents a growing threat for many GNSS-based applications.
Systems involving safety and liability-critical operations (e.g., safe navigation
in ports, systems for smart parking and tolling, GNSS-based synchroniza-
tion of power networks) could potentially be heavily impaired by jamming
attacks. The level of threat associated with jamming cannot be disregarded,
considering that portable jammers are available online and can be purchased
at a very low cost. Although the use of jammers is not legal, the interest of
individuals willing to break the law may result in fraudulent actions toward
GNSS-enabled systems. Several studies have addressed the characterization
of commercial jammers and their effect on GNSS receivers, demonstrating
that they can affect GPS receivers’ functionality even if located up to 9 km
away (see, e.g., [8, 9]). The intentional transmission of a GNSS-like signal is
referred to as spoofing, to distinguish the transmission of specific signals aimed
6519_Book.indb 21 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-26-2048.jpg)
![22 GNSS Interference Threats and Countermeasures
at disrupting the operations of the receivers from the generic introduction of
in-band powerful disturbances. More details about spoofing techniques are
provided in Chapter 3.
1.3 Does Radio-Frequency Interference Exist?
Coffed [10] writes that “Although GPS jamming incidents are relatively rare
they can occur; and, when they do, their impact can be severe.” In fact, nowa-
days topics related to security aspects are very hot in the GNSS community
and very recent publications, even contemporary to the time of this writing
[11], can be found. On February 13, 2014, the Financial Times published an
interview with one of the GPS founders, Professor Bradford Parkinson [12],
on the security of systems relying on GPS. Professor Parkinson clearly recalled
the challenge of making GNSS-based systems more robust. For example, cell
phone towers are often timed with GPS and if they lose their timing reference,
the network loses synchronization with a consequent risk of loss of service.
Professor Parkinson also referred to these concepts during his keynote speech
titled “Assured PNT—Assured World Economic Benefits” at the European
Navigation Conference ENC-GNSS 2014, where he presented his proposal
in response to the GNSS vulnerabilities [13, 14].
The concerns of Prof. Parkinson are shared by many GNSS experts. It
is in fact clear that interference is one of the main limitations to the develop-
ment of GNSS-based applications and services. The threat is relevant when
the interference is unpredictable, because in other cases the receiver can imple-
ment an ad hoc solution for specific interfering sources, as is the case for the
aeronautical bandwidths that are shared with other radio-aiding communi-
cation systems. In recent times, several unexpected interference events have
been reported; for the sake of presenting an example, some of them are briefly
described in the following subsection.
1.3.1 Examples of Real Cases of RF Interference
Some of the literature reports about GPS failures that occurred during trials
and/or experiments in controlled interference scenarios. Many other works
report cases of GPS failures in real situations. Some examples of both testing
results and interference incidents are as follows:
• In January 2007, GPS services were significantly disrupted through-
out San Diego, California [11]. Naval Medical Center emergency
6519_Book.indb 22 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-27-2048.jpg)
![The Interference Threat23
pagers stopped working, the harbor traffic-management system used
for guiding boats failed, airport traffic control had to use backup
systems and processes to maintain air traffic flow, cell phones users
found they had no signal, and bank customers trying to withdraw
cash from automated teller machines (ATMs) were refused. It took 3
days to find an explanation for this mysterious event: Two Navy ships
in San Diego Harbor had been conducting a training exercise when
technicians jammed radio signals. Unwittingly, they also blocked GPS
signals across a broad swath of the city [11].
• A famous incident, well known in the GNSS community, occurred
at Newark Airport, New Jersey, in 2010 when one of the local-area
augmentation system (LAAS) ground facility (LGA) receivers was
occasionally jammed by personal privacy devices (PPDs) installed
onboard vehicles passing along a nearby motorway. In that case, some
of the truck drivers were illegally using a jammer to inherit the GNSS
receiver and hide their trucks’ positions from the truck fleet manager.
The use of GNSS jammers is currently growing in the road domain
and starting to be tackled. This event is also meaningful due to the
effort required to determine that emissions from mobile PPDs were
responsible for the interference at Newark Airport [11, 15].
Eventually, in August 2013, the Federal Communications Commission
(FCC) fined a man nearly $32,000 (Readington, New Jersey) after
concluding he interfered with Newark Liberty International Airport’s
satellite-based tracking system by using an illegal GPS jamming device
in his pickup truck to hide from his employer. The signals emanating
from the vehicle blocked the reception of GPS signals used by the air
traffic control system.
• In January 2011, the U.S. FCC waived restrictions against terrestrial
transmitters in the 1525–1559-MHz band allocated for space-to-Earth
satellite communications. The agency issued an order that allowed
LightSquared Subsidiary LLC to proceed with its plan to deploy a
network of base stations, under the condition that the company form
a working group to look into the GPS interference issue [1, 17]. The
report of the Technical Working Group (TWG) was submitted to the
FCC on June 30, 2011, demonstrating widespread adverse effects by
LightSquared transmissions on all categories of receivers tested [18].
Wideband receivers, in particular, seem to be adversely affected by the
adjacent LightSquared interference; this fact has worried the military
community and the civil high-precision applications stakeholders.
6519_Book.indb 23 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-28-2048.jpg)
![24 GNSS Interference Threats and Countermeasures
• An interesting description of a trial conducted in 2008 on GPS jam-
ming in the maritime sector can be found in [10]. It perfectly highlights
how a GPS denial might strongly affect other onboard equipment.
The experiment was conducted by the General Lighthouse Authori-
ties of the United Kingdom and Ireland (GLAs), in collaboration with
the U.K. government’s Defence Science and Technology Laboratory
(DSTL) at Flamborough Head on the east coast of the United King-
dom. A low-to-medium power jammer, controlled remotely by two
very-high-frequency (VHF) transceivers, transmitted a known pseu-
dorandom noise code over the civilian L1 frequency, which provided
a jamming signal over the whole 2-MHz bandwidth of L1, and a trial
vessel made several runs between two waypoints positioned outside
the jamming area. Authors of [19] outline all of the direct and indirect
effects that the GPS jamming unit had on both the onboard equip-
ment and the reference station. Among the onboard equipment, GPS
and eLoran receivers, automatic identification systems (AIS), digital
selective calling (DSC) systems, and the vessel’s electronic chart dis-
play information system (ECDIS) manifested some malfunction-
ing. Onshore, the differential GPS (DGPS) reference station and the
synchronized lights (conventional aid-to-navigation systems) were
affected by the presence of the jammer.
• Two interference events due to spurious emission of TV transmitters
were detected in 2006. In one case [20, 21], the disturbance, likely due
to digital video broadcasting television (DVB-T) transmitters, was the
cause of performance degradation in the acquisition stage of a GPS
receiver operating in the area, with a consequent loss of the GPS signal
tracking. In the latter case, ultrahigh-frequency (UHF) harmonics
have been detected in Sydney, Australia, around TV antennas. The
undesired signal in the L1 band corrupted the correct performance of
the receiver chain, leading to significant variations in the AGC/ADC
block and in the final user positioning [22].
This list of events is, of course, not exhaustive and further examples of real
cases of interference events can be found, for example, in [23].
1.4 Review of Digital GNSS Receivers
A full description of the GNSS receiver architecture is beyond the scope of
this book. However, we discuss here the main aspects related to the receiver
6519_Book.indb 24 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-29-2048.jpg)
![The Interference Threat25
and to the signal model because such a discussion will prove useful in the
following chapters.
In Figure 1.1 a simplified scheme of the first operational stages of a
GNSS receiver is illustrated. The received signal yRF(t) is composed of the sum
of all received waveforms broadcast by the NS satellites in view at the time of
measurement, noise, and other disturbing signals and can be written as
yRF
(t) = sRF,l
(t)+ i(t)+ n(t)
l=0
NS
−1
∑ (1.1)
where sRF,l(t) is the useful GNSS signal received by the lth satellite in line of
sight, i(t) is the additive interfering signal transmitted over a carrier frequency
fint and characterized by a two-sided bandwidth Bint, and n(t) is the additive
white Gaussian noise.
The front-end block is in charge of demodulating the composite received
signal to an intermediate frequency (IF) and passing it through a filter with
bandwidth BIF to remove the image frequencies. At the output of the ADC/
AGC block of Figure 1.1, composed of the analog-to-digital converter (ADC)
driven by the automatic gain control (AGC), the continuous signal is digi-
tized in yIF(nTS), where Ts = 1/fs is the time sampling interval, and n is the
discrete-time index. Thus, the composite received signal at the ADC/AGC
output can be written as
yIF
[n] = yIF
nTs
( ) = Qk
u
sIF,l
nTs
( )+ iIF
nTs
( )+ h nTs
( )
l=0
L−1
∑
⎡
⎣
⎢
⎤
⎦
⎥ (1.2)
where iIF(t) is the demodulated version of the interfering signal (filtered if Bint
BIF) and η(t) is the filtered Gaussian noise, the function Qu
k denotes the
quantization over k bits, and Ts is the sampling interval. Expanding the term
Figure 1.1 Functional blocks of GNSS receiver.
6519_Book.indb 25 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-30-2048.jpg)
![26 GNSS Interference Threats and Countermeasures
SIF,l(nTs), the expression for the single digitized GNSS signal affected by noise
and interference components becomes (neglecting for the sake of simplicity
the subscript l)
yIF
[n] = Qk
u
2Cd n - n0
⎡
⎣ ⎤
⎦c n - n0
⎡
⎣ ⎤
⎦⋅ cos 2pFD,0
n + j0
( )+ iIF
[n]+ h[n]
⎡
⎣
⎤
⎦ (1.3)
where C is the received GNSS signal power from one satellite in view, d[n]
and c[n] are, respectively, the navigation data message signal and the pseudo-
random noise sequence, FD,0 = (fIF + f0)Ts is the Doppler-affected frequency,
n0 = (τ0/Ts) is the digital code delay, ϕ0 is the instantaneous carrier phase,
and i[n] and η[n] are the digitized interference and the digital Gaussian noise
component, respectively. Given BIF, the front-end bandwidth, it can be shown
that after sampling the signal at the Nyquist frequency fs = 2BIF, the noise
variance becomes
sIF
2
= E h2
[n]
{ } =
N0
fs
2
= N0
BIF
(1.4)
where N0
/2 is the power spectral density (PSD) of the noise.
In the acquisition block, Doppler frequency ˆ
fd
and code phase ˆ
τ
estimations are provided by correlations among the in-phase and quadrature
components of the incoming signal and a GNSS code local replica. More
details about the acquisition procedure are available, for example, in [24, 25]
and are not addressed further in this chapter. The effect of the different types
of interference on the acquisition stages is investigated in Chapter 2.
The signal tracking follows the signal acquisition. Over each channel
of the receiver, a delay lock loop (DLL) is used to synchronize the received
spreading code and a local replica, while a phase lock loop (PLL) is generally
employed to track the phase of the incoming carrier. The signal tracking relies
on the properties of the signal correlation and is fundamental to demodulate
the navigation message and estimate the range between the user and the sat-
ellites. Conventional receiver architectures generally include a frequency lock
loop (FLL) to refine the rough estimate performed by the signal acquisition.
The FLL eases the PLL lock, reducing the transient time between the signal
acquisition and the steady-state carrier/code tracking.
Figure 1.2 shows the block diagram of a tracking system commonly
used in digital GNSS receivers for a single channel, but the same architecture
is repeated over all channels to track different satellites (or different channels
from the same satellite in case of composite signals as foreseen for the Galileo
system).
6519_Book.indb 26 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-31-2048.jpg)
![28 GNSS Interference Threats and Countermeasures
When both the DLL and PLL are locked, the incoming signal is despread
and converted to baseband. The navigation data bits appear at the output of
the in-phase prompt correlator and can be decoded. In addition, with the DLL
locked, the local and the incoming codes are aligned. Referring to the local
code, the receiver knows exactly when a new code period starts and is able to
recognize navigation data bits and boundaries of the navigation message. The
receiver stays synchronized to the tracked satellites, continuously counting
the number of received chips, full code periods, navigation bits, and message
frames. These counters are fundamental to measuring the misalignment over
different channels and tracking different satellites, and are used to compute
the pseudoranges. Once at least four of the pseudoranges are obtained the
position is estimated by means of a trilateration procedure.
In Chapter 2, the impact of the presence of i(t) on the different stages
of the receiver is analyzed, showing the effect on the acquisition probabilities
and on the tracking jitter.
1.5 Organization of the Book
The book is divided into two parts. Chapters 1, 2, and 3 provide an overview
and classification of interference and spoofing sources. The different sources
are discussed in terms of their features (frequency, modulation, and so on) and
their proper model with respect to the GNSS signals. Chapter 4 introduces
some common techniques for the analytical assessment of the interference
effects, and can be used as a reference for the prediction of the performance
of a GNSS receiver in an interfered environment.
The second part of the book is then devoted to describing the techniques
for the detection and mitigation of interference and spoofing attacks. Chap-
ter 5 presents an overview of the common interference detection techniques
tailored to the different families of interference. Mitigation of interference
is addressed by Chapters 6 and 7, presenting classical mitigation techniques
and advanced signal processing techniques, respectively. Chapter 8 discusses
the best strategies for providing antispoofing features to GNSS civil signals.
References
[1] http://www.gps.gov/spectrum/lightsquared/.
[2] Humphreys T., et al., “Assessing the Spoofing Threat: Development of a Portable GPS
Civilian Spoofer,” in Proc. of the 21st International Technical Meeting of the Satellite
6519_Book.indb 28 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-33-2048.jpg)
![The Interference Threat29
Division of the Institute of Navigation (ION GNSS 2008), Savannah, GA, September
2008, pp. 2314–2325.
[3] Yeh, K. C., and C.-H. Liu, “Radio Wave Scintillations in the Ionosphere,” Proc. IEEE,
Vol. 70, No. 4, 1982, pp. 324–360.
[4] Doherty, P. H., et al., “Ionospheric Scintillation Effects in the Equatorial and Auroral
Regions,” Proc. 13th Int. Technical Meeting of the Satellite Division of the Institute of
Navigation (ION GPS 2000), Salt Lake City, UT, pp. 662–671.
[5] Betz, J. W., “Effect of Narrowband Interference on GPS Code Tracking Accuracy,”
Proc. 2000 National Technical Meeting of the Institute of Navigation, Anaheim, CA,
January 2000, pp. 16–27.
[6] Titus, L. B. M., et al., “Intersystem and Intrasystem Interference Analysis Methodol-
ogy,” in Proc. ION GPS/GNSS 2003, Portland, OR, September 2003.
[7] Liu, W., et al., “GNSS RF Compatibility Assessment: Interference Among GPS, Gali-
leo, and Compass,” GPS World, December 2010.
[8] Mitch, R. H., et al., “Civilian GPS Jammer Signal Tracking and Geolocation,” Proc
25th Int. Technical Meeting of The Satellite Division of the Institute of Navigation (ION
GNSS 2012), Nashville, TN, September 2012, pp. 2901–2920.
[9] Borio, D., C. O’Driscoll, and J. Fortuny, “Jammer Impact on Galileo and GPS Receiv-
ers,” Proc. 2013 Int. Conf. on Localization and GNSS (ICL-GNSS), June 25–27, 2013,
pp. 1, 6. doi:10.1109/ICL-GNSS.2013.6577265
[10] Grant, A., and P. Williams, “GNSS Solutions: GPS Jamming and Linear Carrier Phase
Combination,” Inside GNSS, Vol. 4, No. 1, January/February 2009.
[11] Coffed, J., “The Threat of GPS Jamming. The risk to an Information Utility”; available
at http://www.exelisinc.com/solutions/signalsentry/Documents/ThreatOfGPSJam-
ming_February2014.pdf.
[12] Jones S., and Hoyos C.,“GPS Pioneer Warns on Network’s Security,” Financial
Times, http:// http://www.ft.com/cms/s/0/fadf1714-940d-11e3-bf0c-00144feab7de.
html#axzz3J2VEueWr.
[13] Gutierrez, P., “At ENC 2014: A GNSS Wake Up Call for Europe,” Inside GNSS News,
April 16, 2014; available at http://www.insidegnss.com/node/3985.
[14] Jewell, D., “Protect, Toughen, Augment: Words to the Wise from GPS Founder,”
GPS World, April 15, 2014; available at http://gpsworld.com/protect-toughen-
augment-words-to-the-wise-from-gps-founder.
[15] Grabowsky, J. C., “Personal Privacy Jammers. Locating Jersey PPDs Jamming GBAS
Safety-of-Life Signals,” GPS World, Vol. 23, No. 4, April 2012.
[16] Pullen, S., and G. X. Gao, “GNSS Jamming in the Name of Privacy,” Inside GNSS,
Vol. 7, No. 2, March/April 2012.
[17] “LightSquared Fails FCC GPS Interference Tests,” 360 Degrees Column, Inside GNSS,
Vol. 6, No. 4, July/August 2011, pp. 12–15.
6519_Book.indb 29 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-34-2048.jpg)
![30 GNSS Interference Threats and Countermeasures
[18] Boulton, R., et al., “GPS Interference Testing—Lab, Live, and LightSquared,” Inside
GNSS, Vol. 6, No. 4, July/August 2011, pp. 32–45.
[19] Grant, A., et al., “GPS Jamming and the Impact on Maritime Navigation,” Journal of
Navigation, Vol. 62, No. 2, April 2009, pp 173–187.
[20] Motella, B., M. Pini, and F. Dovis, “Investigation on the Effect of Strong Out-of-Band
Signals on Global Navigation Satellite Systems Receivers,” GPS Solutions, Vol. 12, No.
2, March 2008, pp. 77–86.
[21] De Bakker, P., et al., “Effect of Radio Frequency Interference on GNSS Receiver
Output,” Proc. 3rd ESA Workshop on Satellite Navigation User Equipment Technologies
(NAVITEC 2006), ESA/ESTEC, Noordwijk, The Netherlands, December 2006.
[22] Balaei, A. T., B. Motella, and A. G. Dempster, “GPS Interference Detected in Sydney-
Australia,” Proc. 2007 Int. Global Navigation Satellite System (IGNSS 2007) Conf.,
Sydney, Australia, December 2007.
[23] Motella, B., et al., “Assessing GPS Robustness in Presence of Communication Sig-
nals,” Communications Workshops 2009, June 14–18, 2009, pp. 1, 5. doi:10.1109/
ICCW.2009.5207985
[24] Kaplan, E., and C. Hegarty, Understanding GPS Principles and Applications, 2nd ed.,
Norwood, MA: Artech House, 2005.
[25] Misra, P., and P. Enge, Global Positioning System: Signals, Measurements, and Perfor-
mance, Lincoln, MA: Ganga-Jamuna Press, 2006.
6519_Book.indb 30 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-35-2048.jpg)
![Classification of Interfering Sources and Analysis of the Effects on GNSS Receivers33
• Wideband interference (WBI): The spectral occupation is comparable
with respect to the GNSS signal bandwidth (Bint ≈ BGNSS).
• Continuous-wave interference (CWI): Represents the ultimate limit in
NBI and appears as a single tone in the frequency domain (Bint → 0).
Furthermore, in general, interference might have frequency-varying
characteristics, for example, the chirp signals characterized by a linear varia-
tion in time of the instantaneous frequency, thus appearing as WBI. This
kind of interfering signal is typically generated by the jammers. Such devices
are capable of transmitting strong power chirp signals sweeping several
megahertz in a few microseconds, thus obscuring the correct signal recep-
tion of each GNSS channel. Due to their availability on the web, this type
of intentional interfering signal is gaining more and more attention in civil-
ian applications.
CWI could have a severe impact on a GNSS receiver, either on the acqui-
sition or on the tracking process, because the interference power is dispersed on
the whole search space by the correlation with the local code, compromising
the acquisition accuracy and affecting the other functional blocks. The impact
of CWI and NBI strongly depends on the value of the central frequency of
the interference within the frequency band. This is due to the almost periodic
nature of GNSS signals. In fact, the spectrum of a GNSS signal has compo-
nents spaced at multiples of the inverse of the code period (e.g., 1 kHz for GPS
C/A code) with different power allocated to each component depending on
the shape of the code spectrum. The impact of CWI is larger in cases where
the CWI is matched with such components [1–3].
2.2.2 Pulsed Interference
Pulsed interfering signals are characterized by an on–off status of short dura-
tion (order of microseconds), which alternate in the time domain. This type of
interference signal is typical of aviation scenarios, where several aeronautical
radio navigation Services (ARNS) broadcast strong pulsed signals in a band-
width that is shared with some of the satellite navigation systems.
The parameters used to describe pulsed interference are:
• Pulse width (PW): duration of one pulse;
• Pulse repetition frequency (PRF): number of pulses per second;
• Duty cycle (DC = PRF*PW): the percentage of time that is occupied
by the pulses.
6519_Book.indb 33 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-38-2048.jpg)
![34 GNSS Interference Threats and Countermeasures
Pulsed interference with low DC has a small impact on receiver perfor-
mance compared to continuous interference with the same power and center
frequency.
2.3 Potential Interference Sources
The potential interference can share the GNSS frequencies (in-band RFI) or
be far from the GNSS carrier (out-of-band RFI). There are almost no in-band
authorized emissions in the GNSS bandwidths; however, interference comes
mainly from the spurious emissions of out-of-band systems, which generate
harmonics that collide with the GNSS bandwidths.
2.3.1 Out-of-Band Signals
In the following sections, some of the main potential out-of-band interference
sources are analyzed.
Analog TV Channels
TV emissions are veritable sources of interference for a GNSS receiver. They
can manifest as both wideband and narrowband interference: The video car-
riers are considered to be medium/wideband signals, whereas the sound car-
riers are considered to be CWI. In the broadcast TV signal, VHF and UHF
bands are used. The harmonics of such bands generated by TV ground sta-
tion transmitters can generate potentially dangerous interference for GNSS
receivers as depicted in Figure 2.1.
As an example, in [4] a case of interference from a TV signal is reported.
In this case, the interference signal affects the active antenna LNA causing
harmonic distortion in the same LNA that results in an average of 5-dB loss
in C/N0. In [3] six TV channels, French and American equivalents, with their
harmonics are analyzed in frequency and power terms.
DVB-T Signals
The DVB standard has been defined (since 1993) within an initiative involv-
ing more than 300 European and extra-European members. The DVB proj-
ect harmonized the strategies for introducing digital television and the new
multimedia interactive services on transmission networks. It also defined the
technical specifications. The project defined the system specifications for stan-
dard Digital Video Broadcasting–Satellite (DVB-S), developed for the direct
diffusion of TV multiprogramming from satellites and for standard Digital
6519_Book.indb 34 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-39-2048.jpg)
![Classification of Interfering Sources and Analysis of the Effects on GNSS Receivers35
Video Broadcasting–Cable (DVB-C) for the distribution of television signals
through core networks. The DVB family also comprehends the standard for
Digital Video Broadcasting–Terrestrial (DVB-T), for the provision of wire-
less digital terrestrial television. The DVB-T standard is based on the Moving
Pictures Experts Group-2 (MPEG-2) standard for audio/video signal source
coding and it adopts a multi-carrier modulation COFDM to distribute the
total data stream among a large number of carrier frequencies equally spaced
and modulated using QPSK, 16-QAM, 64-QAM, nonuniform 16-QAM, or
nonuniform 64-QAM [5].
Figure 2.1 Potential TV channel harmonic interference.
6519_Book.indb 35 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-40-2048.jpg)
![36 GNSS Interference Threats and Countermeasures
In the European broadcasting area, the DVB-T frequency bands are the
VHF III (174–230 MHz), UHF IV (470–862 MHz), and UHF V (582–862
MHz) bands. These frequency values do not represent a direct threat to GNSS
receivers, but they can cause some problems if harmonics due to potential
distortions caused by the malfunction of some electronic devices, like power
amplifiers, are considered. Even one single damaged amplifier in the amplifi-
cation chain could cause nonlinear behavior, introducing spurious emissions
at the RF output that, due to the high power level emitted, could represent
a real threat for a nearby GNSS receiver. Furthermore, considering that the
frequency involved in the DVB-T signal is the same of that of analog TV,
the probability of having some disturbances caused by DVB-T signals can be
considered similar to that of having spurious emissions from analog television
systems. In [6–8] some examples of significant variations in the quality of the
GPS signal due to analog television transmitters are reported.
Considering, for example, the third harmonics1
of UHF V carrier, it
would fall into the L1 GPS band representing a nonnegligible threat to the
receiver. Therefore, it is important to evaluate the possibility of distortions
caused by nonlinear amplifiers or linear ones in saturation.
A detailed analysis of OFDM DVB-T potential interference in Europe
is reported in [7] where the impact of RFI on the GNSS useful signal is evalu-
ated by means of the spectral separation coefficient.
VHFCOM
Other VHF communication systems can be considered dangerous to a GNSS
receiver [3, 9]. The VHF band (118–137 MHz) contains 760 channels spaced
by 25 kHz, and it is commonly used by air traffic control (ATC) communica-
tions. The harmonics are considered to be NBI with a bandwidth of about 25
kHz. The VHF channels, centered at 121.150, 121.175, and 121.200 MHz,
have the 13th harmonic within the GPS bandwidth, whereas the channels
centered at 131.200, 131.250, and 131.300 MHz are dangerous for their
12th harmonic. In Figure 2.2 VHF communication (VHFCOM) potential
harmonics are depicted.
FM Harmonics
Also small frequency bands inside the FM band (87.5–108 MHz) have har-
monics that fall in the GNSS bands. The channels at 104.9 and 105.1 MHz
have their 15th harmonics near the GPS and Galileo bandwidths as depicted
1
The harmonic order is considered with respect to the signal fundamental frequency (f0) adopting
the definition used in [8].
6519_Book.indb 36 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-41-2048.jpg)
![38 GNSS Interference Threats and Countermeasures
Multi-carrier transmission in a SATCOM service produces intermodulation
products that can fall in the GNSS band. A possible example is reported in [3].
VOR and ILS Harmonics
The VHF omnidirectional range (VOR) is a type of radio navigation system
for aircraft that provides information about radial position with respect to
ground station. The instrument landing system (ILS) consists of two radio
transmitters providing lateral and vertical guidance to aircraft for approach-
ing landing. VOR/ILS emitters are usually positioned at the beginning, end,
and sides of airport runways. These approaching landing systems operate in
the 108–117.95-MHz band including 200 channels frequency spaced at 50
kHz. In detail, the VOR uses 12 channels in the 112.24–112.816-MHz band
while the ILS localizer transponder only uses one frequency on 40 channels
in the 108.10- to 111.95-MHz band. Their harmonics, the 14th from VOR
Figure 2.3 Potential FM harmonic interference.
6519_Book.indb 38 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-43-2048.jpg)
![Classification of Interfering Sources and Analysis of the Effects on GNSS Receivers39
and 2nd from ILS corresponding to 111.9 and 111.95 MHz, enter on the L1/
E1 band. They are considered CWI signals.
Mobile Satellite Service (MSS)
The mobile satellite service (MSS) system can generate two distinct inter-
ference threats to a GNSS receiver. The MSS mobile earth stations use the
1610–1660.5-MHz band, potentially introducing wideband power in the
GNSS band.
Mobile Phone Interference
In general, no direct consequences from mobile phones on a GNSS receiver
have been reported in the literature so far. Some information is available for
aircraft navigation equipment, where a GPS receiver is commonly used. In
[10], an investigation of spurious emissions from six wireless phone technolo-
gies is described, analyzing the effects on aircraft systems, among which is a
GPS. The tests are carried out from semianechoic and reverberation cham-
bers using wireless phone technologies with different transmission frequen-
cies and different receiving antennas. The study is conducted evaluating the
total radiated power from each cellular versus the frequencies of the system
considered. In the analysis, the receiver sensitivity for the GPS receiver is
−120 dBm, but a more realistic level is considered to be around −82 dBm.
This value is obtained considering a minimum path loss of 38 dB. This gap
is evaluated in [11], calculating the path loss after having generated signals
inside the plane. The results show that all the considered values exceed the
receiver system sensitivity level but at the same time are under the more real-
istic value obtained from the path loss. So, the conclusions of the paper are
that the radio-frequency emission from the phones tested do not interfere with
the avionics system examined, among which is the GPS.
2.3.2 In-Band Signals
Some interference sources broadcast signals whose carrier frequency is allocated
in the GNSS bands, and thus they generate in-band interference. Chapter 1
discussed how intersystem and intrasystem interference have to be considered
a primary source of in-band disturbance. However, the level of acceptable
interference is defined during the design phase of the systems, and, so far,
the acceptable level has been the result of international negotiations, discus-
sions, and agreements (consider, e.g., the GPS/Galileo interoperability agree-
ment). In this section the focus is on terrestrial non-GNSS systems. A short
6519_Book.indb 39 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-44-2048.jpg)
![40 GNSS Interference Threats and Countermeasures
description of the most relevant systems emitting power in one or more of the
GNSS bands is provided in the following subsections.
Military/Civil Aeronautical Communication Systems
The military communications systems can be considered in-band interferers
due to the signal band used by the systems. The Galileo E5a and E5b bands,
located within 1164–1214 MHz, occupy frequencies already used for aero-
nautical radio-navigation services (ARNS) such as for tactical air navigation
(TACAN), distance measuring equipment (DME), and secondary surveillance
radar (SSR), as well as by the DoD Joint Tactical Information Distribution
System (JTIDS) and the Multifunction Information Distribution System
(MIDS). Other aeronautical systems operate in these frequencies such as the
Traffic Collision and Avoidance System (TCAS), Identification Friend or Foe
(IFF), and planned Automatic Dependent Surveillance–Broadcast (ADS-B).
The DME/TACAN systems consist of an airborne interrogator and a
ground-based transponder that emits high-power pulsed signals that constitute
a threat to GNSS receivers. DME and TACAN provide range measurements
of the aircraft with respect to a ground reference station. The TACAN is a
military system that provides range and azimuth measurements. The DME/
TACAN system operates in the 960- to 1215-MHz frequency band [12] in
four different modes: X, Y, W, and Z, even if only the DME/TACAN ground
transponder X mode occupies the 1151–1215 MHz frequency band that inter-
feres with the E5a/L5 and E5b GNSS signal (see Figure 2.4 and Table 2.1).
The analytical expression of the classical DME/TACAN pulse pair
transmitted by the ground beacons is
ypulse
(t) = e
−
a
2
⎛
⎝
⎜
⎞
⎠
⎟t2
+ e
−
a
2
⎛
⎝
⎜
⎞
⎠
⎟(t−∆t)2
(2.1)
where, for example, for the X mode α = 4.5 ⋅ 1011
s−2
and the interpulse inter-
val is Δt = 12 μs.
The maximum pulse repetition frequency (PRF) for the DME and the
TACAN system are 2700 and 3600 ppps, respectively.
JTID/MIDS are spread-spectrum digital communications systems for
exchanging data among military platforms. They operate between 969 and
1206 MHz interfering with the E5a/E5b bands as reported in Figure 2.5.
Ultra-Wideband Signals
The definition of UWB includes any signal that occupies more than 500 MHz
between 3.1 and 10.6 GHz and meets the spectrum mask that defines the
6519_Book.indb 40 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-45-2048.jpg)
![Classification of Interfering Sources and Analysis of the Effects on GNSS Receivers43
[13–15], concluded that UWB signals can degrade GPS receiver performance.
Other studies [16, 17] demonstrate by simulation and in a wireless personal
area network (WPAN) case study, respectively, that UWB interference effect
can be reduced by choosing the proper modulation parameters.
2.3.3 Classification of Jammers
As explained in Chapter 1, the term jamming refers to intentional transmis-
sion of RF interference with the goal of masking certain portions of frequency
bands with noise. In the case of GNSS systems, a jammer (also referred to as
a personal privacy device (PPD)) is able to jam (or block) GNSS signals, likely
preventing the receivers from operating correctly within the jammer area. As
an example, Figure 2.6 shows two different models of jammers. Both of them
are able to transmit over different frequency bands, including the GNSS E1/
L1 band.
Intentional interference is a well-known threat in military applications,
but it is also considered a growing concern in the civil environment, thanks
Figure 2.6 Examples of multifrequency GNSS jammers: adjustable desktop jammer
(left) and four-antenna portable device (right).
6519_Book.indb 43 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-48-2048.jpg)
![44 GNSS Interference Threats and Countermeasures
to the fact that real incidents caused by PPDs have already occurred (consider
the incident at the Newark Airport in New Jersey, described in Chapter 1).
It is worth recalling that in many countries (e.g., United States or several
European countries) jammers are illegal to sell or use. In spite of this, it might
not be forbidden to own or buy a jammer, easily achievable through several
websites, even at a cost of few tens of dollars [18].
Next, we summarize the main classifications of jammers proposed in
the literature and discuss their main characteristics.
A survey of jammers, specifically tailored to in-car jammers, is proposed
in [19]. In-car jammers are small devices, powered by a car’s cigarette lighter.
This class of jammers is particularly important, because their use might allow
users (e.g., vehicles) to avoid being tracked.
In [19] jammers are classified into four classes depending on their signal
characteristics: A few of them transmit a continuous-wave (CW) signal, while
the majority use a chirp signal. The signal bandwidth varies from less than
1 kHz (for the case of CW) to 44.9 MHz, with a sweep time in the interval
[8.62 ÷ 18.97] μs.
A further classification of jammers can be found in [20], where the
categories are mainly based on power source. All the jammers analyzed in
[20] are portable devices, divided into three groups: devices designed to plug
into a car cigarette lighter’s 12-volt supply (Group 1), and devices powered
by an internal rechargeable battery with (Group 2) or without (Group 3) an
external antenna. As a consequence of the analysis of 18 different devices,
the authors concluded that all of them use a swept tone method to generate
broadband interference on the L1 or L2 band (with a sweep rate of 9 μs, on
average, covering a bandwidth of 20 MHz). They also provided an estimate
of the analyzed jammers’ effective ranges, which vary in the [300m ÷ 6 km]
range for tracking, and the [600m ÷ 8.5 km] range for acquisition.
A further survey of jammers can be found in [21], where multifrequency
jammers, able to simultaneously disturb more than one GNSS band (L1, L2,
and L5), are characterized. The analysis confirmed that cigarette lighter jam-
mers only operate on the L1 band, with different values for the sweep period
(with 9 μs being the most common value). It was also shown that the transmit-
ted power can vary from −10 to more than 30 dBm and, in general, cigarette
lighter jammers are characterized by lower power levels than multifrequency
battery jammers.
An example of a chirp signal generated by a portable jammer device is
depicted in Figure 2.7. The figure shows the time-frequency representation of
the signal emitted by the jammer. It can be seen that the chirp signal sweeps
approximately 9 MHz during an interval of 10 μs.
6519_Book.indb 44 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-49-2048.jpg)
![48 GNSS Interference Threats and Countermeasures
Figure 2.8 (Continued) (c) GPS L1 C/A code PSD in the presence of CWI. (d)
Histograms of the samples at the ADC output in presence of CWI.
The correlation with the local code can be seen as a spreading operation fol-
lowed by a filter.
In [22] an exhaustive derivation of the impact of CWI on the acquisi-
tion stage of a GNSS receiver is provided. In the presence of CWI the expres-
sion of the digitized signal2
at the input of the baseband processing block of
a GNSS receiver is
2
For the sake of simplicity, in this description the effect of the data and of the quantization effects
are neglected.
6519_Book.indb 48 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-53-2048.jpg)
![Classification of Interfering Sources and Analysis of the Effects on GNSS Receivers49
yIF
[n] = 2Cc n - t0
⎡
⎣ ⎤
⎦cos 2p fIF
+ fD,0
( )Ts
n + j0
( )
+Aint
cos 2pFint
Ts
n + qint
( )+ h[n] (2.2)
where the first member of the sum is the useful received GNSS signal; Aint,
Fint, and θint are, respectively, the amplitude, the carrier frequency, and the
random phase uniformly distributed in the range [−π ;π) of the CWI assumed
to be a pure sinusoid; and WIF[n] is the Gaussian noise component that, under
the Nyquist sampling theorem assumption, can be assumed to be a classical
independent and identically distributed (IID) discrete random process.
According to the equivalent scheme of a GNSS acquisition block depicted
in Figure 2.9, the signal in (2.2) is first multiplied by a complex exponential and
then multiplied and integrated with respect to the local code chosen according
to an hypothesis of Doppler frequency fD and code delay τ, thus providing
the following cross ambiguity function complex components:
SI
t, fD
( ) =
1
N
rI
[n]c[n − t] = rI
[t]∗ hc
[t]
n=0
N−1
∑
SQ
t, fD
( ) =
1
N
rQ
[n]c[n − t] = rQ
[t]∗ hc
[t]
n=0
N−1
∑ (2.3)
where hc is the equivalent filter representing the operation of multiplication
and integration with respect the local code, and N is the number of coherent
integrated samples. Finally the CAF is obtained as a complex modulus
Figure 2.9 Equivalent scheme of a GNSS acquisition block.
6519_Book.indb 49 12/19/14 3:37 PM](https://image.slidesharecdn.com/4976558-250523070108-2d147bd2/75/Gnss-Interference-Threats-And-Countermeasures-Fabio-Dovis-54-2048.jpg)
Gnss Interference Threats And Countermeasures Fabio Dovis
- 1. Gnss Interference ThreatsAnd Countermeasures Fabio Dovis download https://ebookbell.com/product/gnss-interference-threats-and- countermeasures-fabio-dovis-9953116 Explore and download more ebooks at ebookbell.com
- 2. Here are somerecommended products that we believe you will be interested in. You can click the link to download. Adaptive Interference Mitigation In Gnss 1st Ed 2018 Renbiao Wu https://ebookbell.com/product/adaptive-interference-mitigation-in- gnss-1st-ed-2018-renbiao-wu-6755734 Gnss Software Receivers Kai Borre Ignacio Fernndezhernndez https://ebookbell.com/product/gnss-software-receivers-kai-borre- ignacio-fernndezhernndez-46872968 Gnss Global Navigation Satellite Systems Gps Glonass Galileo And More Bernhard Hofmannwellenhof https://ebookbell.com/product/gnss-global-navigation-satellite- systems-gps-glonass-galileo-and-more-bernhard-hofmannwellenhof-2343752 Gnss For Vehicle Control David M Bevly https://ebookbell.com/product/gnss-for-vehicle-control-david-m- bevly-2528554
- 3. Gnss Remote SensingTheory Methods And Applications 1st Edition Shuanggen Jin https://ebookbell.com/product/gnss-remote-sensing-theory-methods-and- applications-1st-edition-shuanggen-jin-4380692 Gnssr Earth Remote Sensing From Smallsats Hugo Carrenoluengo https://ebookbell.com/product/gnssr-earth-remote-sensing-from- smallsats-hugo-carrenoluengo-54691714 Gnss Space Weather And Tec Special Features Serdjo Kos Jos Fernndez https://ebookbell.com/product/gnss-space-weather-and-tec-special- features-serdjo-kos-jos-fernndez-54692072 Gnss Environmental Sensing Revolutionizing Environmental Monitoring Awange https://ebookbell.com/product/gnss-environmental-sensing- revolutionizing-environmental-monitoring-awange-6751930 Gnss Systems And Engineering The Chinese Beidou Navigation And Position Location Satellite 1st Edition Shusen Tan https://ebookbell.com/product/gnss-systems-and-engineering-the- chinese-beidou-navigation-and-position-location-satellite-1st-edition- shusen-tan-7034728
- 6. GNSS Interference Threats andCountermeasures 6519_Book.indb 1 12/19/14 3:37 PM
- 7. For a listingof recent titles in the Artech House GNSS Technology and Applications Series, turn to the back of this book. 6519_Book.indb 2 12/19/14 3:37 PM
- 8. GNSS Interference Threats andCountermeasures Fabio Dovis Editor artechhouse.com 6519_Book.indb 3 12/19/14 3:37 PM
- 9. Library of CongressCataloging-in-Publication Data A catalog record for this book is available from the U.S. Library of Congress British Library Cataloguing in Publication Data A catalog record for this book is available from the British Library. ISBN-13: 978-1-60807-810-3 Cover design by John Gomes © 2015 Artech House 685 Canton Street Norwood, MA 02062 All rights reserved. Printed and bound in the United States of America. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher. All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized. Artech House cannot attest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark. 10 9 8 7 6 5 4 3 2 1 6519_Book.indb 4 12/19/14 3:37 PM
- 10. To my familyand friends 6519_Book.indb 5 12/19/14 3:37 PM
- 11. 6519_Book.indb 6 12/19/143:37 PM
- 12. 7 Contents Preface 13 Acknowledgments 15 1The Interference Threat 17 1.1 Introduction to the Book 17 1.2 What Is Interference? 19 1.2.1 Natural Sources of Interference 19 1.2.2 Multipath 20 1.2.3 Intersystem and Intrasystem Interference 20 1.2.4 Artificial Interference: Unintentional and Intentional Interference 21 1.3 Does Radio-Frequency Interference Exist? 22 1.3.1 Examples of Real Cases of RF Interference 22 1.4 Review of Digital GNSS Receivers 24 1.5 Organization of the Book 28 References 28 6519_Book.indb 7 12/19/14 3:37 PM
- 13. 8 GNSS InterferenceThreats and Countermeasures 2 Classification of Interfering Sources and Analysis of the Effects on GNSS Receivers 31 2.1 Introduction 31 2.2 Classification of Interfering Source 32 2.2.1 Interference Spectral Features 32 2.2.2 Pulsed Interference 33 2.3 Potential Interference Sources 34 2.3.1 Out-of-Band Signals 34 2.3.2 In-Band Signals 39 2.3.3 Classification of Jammers 43 2.4 The Impact of RFI on GNSS Receivers 45 2.4.1 Impact on the Front-End 46 2.4.2 Impact on the Acquisition Stage 47 2.4.3 Impact on the Tracking Stage 55 2.4.4 Impact on the Estimated Signal-to-Noise Ratio 63 2.5 Conclusions 64 References 64 3 The Spoofing Menace 67 3.1 Introduction: Meaconing and Spoofing Attacks 67 3.2 Meaconing 70 3.3 Spoofing 71 3.3.1 Simplistic Attack 72 3.3.2 Intermediate Attack 76 3.3.3 Sophisticated Spoofers 78 3.4 Hybrid/Combined Spoofing Techniques 80 3.4.1 Relaying Attack 80 3.4.2 Meaconing with Variable Delay 81 3.4.3 Security Code Estimation and Replay Attack 82 3.4.4 Meaconing or Spoofing Plus High-Gain Antennas 83 3.5 Conclusions 84 References 85 4 Analytical Assessment of Interference on GNSS Signals 89 4.1 Introduction 89 4.2 Theoretical Model of the C/N0 Loss in the Presence of Interference 90 6519_Book.indb 8 12/19/14 3:37 PM
- 14. Contents9 4.2.1 Theoretical PulseBlanking Impact on C/N0 Degradation: Pulsed Interference 92 4.3 Spectral Separation Coefficient 94 4.4 The Interference Error Envelope 97 4.5 Conclusions 102 References 102 5 Interference Detection Strategies 105 5.1 Introduction 105 5.2 Interference Detection via AGC Monitoring 108 5.2.1 The Role of the ADC 109 5.3 Interference Detection via Time-Domain Statistical Analysis 111 5.4 Interference Detection via Spectral Monitoring 113 5.5 Interference Detection via Postcorrelation Statistical Analysis 116 5.6 Interference Detection via Carrier-to-Noise Power Ratio Monitoring 119 5.7 Interference Detection via Pseudorange Monitoring 121 5.8 Interference Detection via PVT Solution Observation 122 5.9 Conclusions 123 References 123 6 Classical Digital Signal Processing Countermeasures to Interference in GNSS 127 6.1 Frequency-Domain Techniques 128 6.1.1 Frequency-Domain Adaptive Filtering 128 6.1.2 Notch Filtering 132 6.1.3 Adaptive Notch Filter 133 6.2 Time-Domain Techniques 136 6.2.1 Pulse Blanking Technique 136 6.3 Space-Time Domain Techniques 141 6.3.1 Space-Time Adaptive Processing Techniques 142 6.3.2 Subspace Decomposition for Spatial Filtering 145 6.4 Conclusions 146 References 147 6519_Book.indb 9 12/19/14 3:37 PM
- 15. 10 GNSS InterferenceThreats and Countermeasures 7 Interference Mitigation Based on Transformed Domain Techniques 149 7.1 Introduction 149 7.2 Transformed Domain Techniques 150 7.3 Time-Frequency Representation 152 7.4 Time-Scale Domain: The Wavelet Transform 153 7.4.1 The Discrete Time Wavelet Transform 155 7.4.2 Wavelet Packet Decomposition Based Mitigation Algorithm 156 7.4.3 WPD-Based Method: Parameter Tuning 158 7.4.4 Computational Complexity 161 7.5 Subspace Domain: The Karhunen-Loève Transform 162 7.5.1 KLT Interference Detection and Suppression Algorithm 163 7.6 Case Study: A Pulsed Interference Environment 164 7.6.1 WPD Applied to Pulsed Interference 165 7.6.2 KLT Applied to Pulsed Interference 169 7.6.3 TD Techniques Versus Pulse Blanking: Performance Comparison 170 7.7 Transformed Domain Techniques: Possible Implementation 175 7.8 Conclusions 176 References 177 8 Antispoofing Techniques for GNSS 179 8.1 Introduction 179 8.2 GNSS Receiver Stand-Alone Techniques 180 8.2.1 Consistency Check of Receiver Measurements 181 8.2.2 Signal Quality Monitoring 183 8.3 Hybrid Positioning Receiver Techniques 187 8.3.1 Integration with Inertial Systems 187 8.3.2 Integration with Communication Systems 189 8.4 Authentication Techniques 190 8.4.1 Navigation Message Authentication 191 8.4.2 Spreading Code Authentication 193 8.4.3 Navigation Message Encryption 194 6519_Book.indb 10 12/19/14 3:37 PM
- 16. Contents11 8.4.4 Spreading CodeEncryption 194 8.5 Conclusions 197 References 197 About the Authors 201 Index 205 6519_Book.indb 11 12/19/14 3:37 PM
- 17. 6519_Book.indb 12 12/19/143:37 PM
- 18. 13 Preface Writing a bookis always a battle between completeness and affordability of the text. This is even more true for a topic such as interference in satellite navigation systems—to provide complete coverage would require detailed descriptions of the theory, of the receiver architectures, and of each method that has been implemented or proposed so far. The approach followed by the authors in this book has been to find a balance between the two extremes, providing the readers with a fairly complete overview of the different topics, including a good list of references, but at the same time offering insight into the most promising or innovative techniques. The topic of interference threats is quite hot and new countermeasures are still being proposed, and thus the book focuses on the principles of the methods but avoids providing information that the interested reader can find in the papers included in the references. Although the first chapter provides some basic principles of satellite navigation receivers, the book is intended for members of the engineering/ scientific community who have preexisting knowledge of satellite navigation principles and global navigation satellite systems. We hope that this book will help engineers and scientists to better under- stand the interference and spoofing threats, which in turn will help them to design and implement improved robust systems that are able to handle these menaces. 6519_Book.indb 13 12/19/14 3:37 PM
- 19. 6519_Book.indb 14 12/19/143:37 PM
- 20. 15 Acknowledgments When almost 15years ago I started investigating satellite navigation together with my colleague and friend Paolo Mulassano, I was definitely not expect- ing that one day in the future, I would have the pleasure of being the editor of a book on satellite navigation topics. For achievement of this milestone I have to thank many people who along the years have given me guidance: col- leagues, friends, and students in the NavSAS group. First of all my grateful thanks go to Prof. Letizia Lo Presti, who taught to all of us the passion for research work and the spirit needed to be an effective team. My appreciation is extended in particular to the contributors to this work—Emanuela, Beatrice, Marco, Davide, and Luciano: without your commitment we could have not accomplished the task. Fabio Dovis January 2015 6519_Book.indb 15 12/19/14 3:37 PM
- 21. 6519_Book.indb 16 12/19/143:37 PM
- 22. 17 1 The Interference Threat FabioDovis 1.1 Introduction to the Book Reliable positioning and navigation are becoming imperative in more and more applications related to public services, consumer products, and safety-critical situations. Research aimed at finding pervasive and robust positioning meth- odologies is critical to a growing number of societal areas. Such research also needs to ensure that the navigation is trustworthy and the risks and threats, especially toward satellite navigation, are accounted for. Modern society is highly reliant on global navigation satellite systems (GNSS) and satellite and radio navigation are evolving at an accelerating pace. With the growth of a new European satellite navigation system, Galileo, the development of the Chinese Beidou system, and the modernization of the currently existing systems such as the American GPS or the Russian GLONASS, a wider range of new signals will guarantee better performance, enabling a plethora of new applications. In fact, nowadays, in addition to the obvious usage in position- ing and navigation, more and more applications are relying on a robust timing reference from GNSS. However, although GNSS technology can provide accurate and global positioning, velocity, and time estimations, it is highly vulnerable to a range 6519_Book.indb 17 12/19/14 3:37 PM
- 23. 18 GNSS InterferenceThreats and Countermeasures of threats. GNSS is particularly prone to unintended and malicious radio- frequency interference (RFI) due to the extremely low power level of the signal at the user’s receiver after traveling from the satellite transmitter to the receiving antenna on the Earth. Due to the weakness of the GNSS signal that reaches users and a crowded frequency spectrum, GNSS-based services will be always vulnerable to the presence of interfering signals generated by other communication systems. A recent example of these risks was the LightSquared case in the United States, where the GPS receiver operations in the L1 GPS band have been seriously threatened [1]. Furthermore, GNSS threats include intentional attacks with the objec- tive of disrupting the target receiver. Recalling that GNSS bandwidths are protected, the malicious transmission of counterfeit GNSS-like signals, usu- ally known as spoofing, may become quite dangerous also for civil use of GNSS. Spoofing and GNSS receiver deception are becoming a threat, as more applications and infrastructures begin to rely on GNSS position and time information. Although the vulnerability of GNSS-based civilian infra- structures is understood, few recognize that severe attacks can be carried out with self-made spoofing devices composed of a software receiver and trivial RF front-ends, as recently demonstrated in the United States by researchers at the University of Austin in Texas [2]. Provision of timing references for communication networks, agriculture, fishery, and road tolling applications are just a few examples of markets that would be deeply affected by spoofing activity designed to elude public authorities or service providers. Thus, with the growth of civilian GNSS use, unintentional interference, jamming, and spoofing are emerging security challenges in the civil field. There are several applications for which it is essential to detect such types of intentional deception in order to ensure reliable position and time estimations. The provision of such robustness can protect personal safety or infrastructures such as power grids, distribution networks, or communication networks for which GNSS is the provider of timing information. The importance of ensur- ing a robust receiver with respect to interference and spoofing is crucial for all types of applications where the concept of security is needed. Hence, evaluat- ing the possible impact of potential threats on particular services related to transportation applications (aviation, maritime, railway, road), to emergency applications oriented to the tracking and tracing of sensitive material (e.g., medical or dangerous goods), and to financial/assurance aspects is a priority. The goal of this book is to provide an overview of the major sources of interference and spoofing for a GNSS receiver, discussing both the methods used to assess their impact on the positioning performance as well as the meth- ods used to protect civilian use of GNSS against unintentional and intentional 6519_Book.indb 18 12/19/14 3:37 PM
- 24. The Interference Threat19 attacks.This book introduces methods for detection (and possibly mitigation) of intentional and unintentional interference as well as spoofing countermea- sures, The techniques investigated in this book have advanced primarily as a result of the increased computational capabilities of GNSS receivers, which allow the implementation of more sophisticated signal processing algorithms with respect to the past. Chipset-based, programmable hardware-based, and fully software-based GNSS receivers are also able to host more complex algo- rithms for interference mitigation purposes in cases in which it is desirable to mitigate the effect of the interference without discarding the measurements performed. Such algorithms may work at the raw signal sample level, which allows for timely elaboration of warnings and better observability of the phe- nomenon. The development of innovative algorithms aims at improving the defense mechanisms of several applications and infrastructures with respect to malicious attacks. 1.2 What Is Interference? It is well known that several phenomena may affect the quality of the pseudor- ange estimation that is based on the measurement of the propagation time of a signal from a satellite to the user. Any electromagnetic source interacting with the signals is interfering with the process of estimating the propagation time. This book focuses on artificial sources of RFI generated either intentionally or unintentionally by some communication system. The following chapters address such sources of artificial interference and the receiver-based techniques used to detect and mitigate their effects. Note, however, that other kinds of interference might be a threat to GNSS positioning performance. They are discussed in the following subsections, but are not be specifically addressed in this book since their detection and mitigation follows specific approaches. 1.2.1 Natural Sources of Interference When considering the propagation of a signal in the atmosphere, the effect of the ionosphere has to be taken into account due to its impact on the propa- gation time of the signal. Electron concentration in the ionosphere affects GNSS signals by introducing delays in their propagation. Such errors can be corrected in part by making use of models of the background ionosphere when performing single-frequency measurements, or corrected entirely in the case of dual-frequency measurements. However, in some cases electron den- sity irregularities may appear that can further disrupt the propagation of the 6519_Book.indb 19 12/19/14 3:37 PM
- 25. 20 GNSS InterferenceThreats and Countermeasures wave by introducing fluctuations in amplitude and phase; such phenomena are usually called scintillations [3]. How often GNSS signals are affected by scintillations depends on solar and geomagnetic activity, geographic location, season, local time, and signal frequency. Scintillation can be considered a sort of natural interference interacting with the GNSS signal that causes the signals to fade and induces a frequency shift in the signal carrier that in some cases can strongly affect the GNSS receiver. During strong ionospheric events, amplitude fades and frequency variations can be very challenging for a receiver and may cause frequent cycle slips and losses of lock of the satellite signals [4]. 1.2.2 Multipath Multipath occurs whenever the user device receives reflected signals in addition to the direct line-of-sight signal. These replicas of the signals are generated from the ground, buildings, or trees in terrestrial navigation, whereas signal reflec- tions from the host-vehicle body are more common in airborne and marine applications. Multipath can be specular when generated from smooth surfaces or diffuse when arising from diffuse scatterers and sources of diffraction. To a certain extent, multipath can then be considered a self-interference, where the interfering signal is a replica of the signal itself. 1.2.3 Intersystem and Intrasystem Interference The signal impinging the GNSS receiver antenna at a given frequency is the combination of the signals broadcast by all the satellites in view. GNSS RF compatibility addresses the issue of intrasystem (from the same system) and intersystem (from other systems) interference. Signals belonging to the same satellite constellation are designed to be theoretically orthogonal (exploiting code or frequency diversity), and thus they can be separated by the receiver processing. However, such orthogonality is not perfect and a residual power is always generating intrasystem interference. Intersystem interference is due to the fact that several GNSS systems share the same carriers, and again, some power from the signals of another system can disrupt the signal of interest. Several methodologies, such as the effective carrier power to noise density theory introduced in [5], are used during the design phase of the systems to ensure that a maximum acceptable level of intersystem interference is respected (see, e.g., [6, 7]). Intra- and intersystem interference is then a topic that needs to be addressed during the design phase, and it is beyond the regular users’ capabilities to deal with it. 6519_Book.indb 20 12/19/14 3:37 PM
- 26. The Interference Threat21 Dueto the growing number of operational satellites in the new GNSS constellations, the number of satellites that are in view to a user receiver at the same time is growing as well. This implies increased intersystem interfer- ence. However, from a user’s standpoint, it can only be reduced by means of directional antennas that can spatially filter the signals coming from satellites that are not of interest. 1.2.4 Artificial Interference: Unintentional and Intentional Interference The intrinsic power weakness of GNSS signals affects the performance of any type of receiver, since all the communication systems transmitting at carrier frequencies close to the band of interest are potential sources of interference for a GNSS receiver, and even small leakages out of their allocated bandwidth can be threatening to GNSS signals. Even though unintentional RFI events are generally unpredictable, their presence has been experienced in the past and the increasing number of wireless communication infrastructures is increasing the probability that some power spillover from signal frequencies located near the GNSS bands could affect the performance of GNSS receivers in a certain region. The presence of interfering power can be due to several reasons, but the main effects are caused by harmonics or spurious components generated by intermodulation products in the communication transmitter. Jamming refers to intentional transmission of RF energy to hinder a navigation service by masking GNSS signals with noise. The malicious objec- tive of jammers is to cause the receiver to lose tracking and to impede signal reacquisition. Although jamming is a well-known threat in the military appli- cations, it represents a growing threat for many GNSS-based applications. Systems involving safety and liability-critical operations (e.g., safe navigation in ports, systems for smart parking and tolling, GNSS-based synchroniza- tion of power networks) could potentially be heavily impaired by jamming attacks. The level of threat associated with jamming cannot be disregarded, considering that portable jammers are available online and can be purchased at a very low cost. Although the use of jammers is not legal, the interest of individuals willing to break the law may result in fraudulent actions toward GNSS-enabled systems. Several studies have addressed the characterization of commercial jammers and their effect on GNSS receivers, demonstrating that they can affect GPS receivers’ functionality even if located up to 9 km away (see, e.g., [8, 9]). The intentional transmission of a GNSS-like signal is referred to as spoofing, to distinguish the transmission of specific signals aimed 6519_Book.indb 21 12/19/14 3:37 PM
- 27. 22 GNSS InterferenceThreats and Countermeasures at disrupting the operations of the receivers from the generic introduction of in-band powerful disturbances. More details about spoofing techniques are provided in Chapter 3. 1.3 Does Radio-Frequency Interference Exist? Coffed [10] writes that “Although GPS jamming incidents are relatively rare they can occur; and, when they do, their impact can be severe.” In fact, nowa- days topics related to security aspects are very hot in the GNSS community and very recent publications, even contemporary to the time of this writing [11], can be found. On February 13, 2014, the Financial Times published an interview with one of the GPS founders, Professor Bradford Parkinson [12], on the security of systems relying on GPS. Professor Parkinson clearly recalled the challenge of making GNSS-based systems more robust. For example, cell phone towers are often timed with GPS and if they lose their timing reference, the network loses synchronization with a consequent risk of loss of service. Professor Parkinson also referred to these concepts during his keynote speech titled “Assured PNT—Assured World Economic Benefits” at the European Navigation Conference ENC-GNSS 2014, where he presented his proposal in response to the GNSS vulnerabilities [13, 14]. The concerns of Prof. Parkinson are shared by many GNSS experts. It is in fact clear that interference is one of the main limitations to the develop- ment of GNSS-based applications and services. The threat is relevant when the interference is unpredictable, because in other cases the receiver can imple- ment an ad hoc solution for specific interfering sources, as is the case for the aeronautical bandwidths that are shared with other radio-aiding communi- cation systems. In recent times, several unexpected interference events have been reported; for the sake of presenting an example, some of them are briefly described in the following subsection. 1.3.1 Examples of Real Cases of RF Interference Some of the literature reports about GPS failures that occurred during trials and/or experiments in controlled interference scenarios. Many other works report cases of GPS failures in real situations. Some examples of both testing results and interference incidents are as follows: • In January 2007, GPS services were significantly disrupted through- out San Diego, California [11]. Naval Medical Center emergency 6519_Book.indb 22 12/19/14 3:37 PM
- 28. The Interference Threat23 pagersstopped working, the harbor traffic-management system used for guiding boats failed, airport traffic control had to use backup systems and processes to maintain air traffic flow, cell phones users found they had no signal, and bank customers trying to withdraw cash from automated teller machines (ATMs) were refused. It took 3 days to find an explanation for this mysterious event: Two Navy ships in San Diego Harbor had been conducting a training exercise when technicians jammed radio signals. Unwittingly, they also blocked GPS signals across a broad swath of the city [11]. • A famous incident, well known in the GNSS community, occurred at Newark Airport, New Jersey, in 2010 when one of the local-area augmentation system (LAAS) ground facility (LGA) receivers was occasionally jammed by personal privacy devices (PPDs) installed onboard vehicles passing along a nearby motorway. In that case, some of the truck drivers were illegally using a jammer to inherit the GNSS receiver and hide their trucks’ positions from the truck fleet manager. The use of GNSS jammers is currently growing in the road domain and starting to be tackled. This event is also meaningful due to the effort required to determine that emissions from mobile PPDs were responsible for the interference at Newark Airport [11, 15]. Eventually, in August 2013, the Federal Communications Commission (FCC) fined a man nearly $32,000 (Readington, New Jersey) after concluding he interfered with Newark Liberty International Airport’s satellite-based tracking system by using an illegal GPS jamming device in his pickup truck to hide from his employer. The signals emanating from the vehicle blocked the reception of GPS signals used by the air traffic control system. • In January 2011, the U.S. FCC waived restrictions against terrestrial transmitters in the 1525–1559-MHz band allocated for space-to-Earth satellite communications. The agency issued an order that allowed LightSquared Subsidiary LLC to proceed with its plan to deploy a network of base stations, under the condition that the company form a working group to look into the GPS interference issue [1, 17]. The report of the Technical Working Group (TWG) was submitted to the FCC on June 30, 2011, demonstrating widespread adverse effects by LightSquared transmissions on all categories of receivers tested [18]. Wideband receivers, in particular, seem to be adversely affected by the adjacent LightSquared interference; this fact has worried the military community and the civil high-precision applications stakeholders. 6519_Book.indb 23 12/19/14 3:37 PM
- 29. 24 GNSS InterferenceThreats and Countermeasures • An interesting description of a trial conducted in 2008 on GPS jam- ming in the maritime sector can be found in [10]. It perfectly highlights how a GPS denial might strongly affect other onboard equipment. The experiment was conducted by the General Lighthouse Authori- ties of the United Kingdom and Ireland (GLAs), in collaboration with the U.K. government’s Defence Science and Technology Laboratory (DSTL) at Flamborough Head on the east coast of the United King- dom. A low-to-medium power jammer, controlled remotely by two very-high-frequency (VHF) transceivers, transmitted a known pseu- dorandom noise code over the civilian L1 frequency, which provided a jamming signal over the whole 2-MHz bandwidth of L1, and a trial vessel made several runs between two waypoints positioned outside the jamming area. Authors of [19] outline all of the direct and indirect effects that the GPS jamming unit had on both the onboard equip- ment and the reference station. Among the onboard equipment, GPS and eLoran receivers, automatic identification systems (AIS), digital selective calling (DSC) systems, and the vessel’s electronic chart dis- play information system (ECDIS) manifested some malfunction- ing. Onshore, the differential GPS (DGPS) reference station and the synchronized lights (conventional aid-to-navigation systems) were affected by the presence of the jammer. • Two interference events due to spurious emission of TV transmitters were detected in 2006. In one case [20, 21], the disturbance, likely due to digital video broadcasting television (DVB-T) transmitters, was the cause of performance degradation in the acquisition stage of a GPS receiver operating in the area, with a consequent loss of the GPS signal tracking. In the latter case, ultrahigh-frequency (UHF) harmonics have been detected in Sydney, Australia, around TV antennas. The undesired signal in the L1 band corrupted the correct performance of the receiver chain, leading to significant variations in the AGC/ADC block and in the final user positioning [22]. This list of events is, of course, not exhaustive and further examples of real cases of interference events can be found, for example, in [23]. 1.4 Review of Digital GNSS Receivers A full description of the GNSS receiver architecture is beyond the scope of this book. However, we discuss here the main aspects related to the receiver 6519_Book.indb 24 12/19/14 3:37 PM
- 30. The Interference Threat25 andto the signal model because such a discussion will prove useful in the following chapters. In Figure 1.1 a simplified scheme of the first operational stages of a GNSS receiver is illustrated. The received signal yRF(t) is composed of the sum of all received waveforms broadcast by the NS satellites in view at the time of measurement, noise, and other disturbing signals and can be written as yRF (t) = sRF,l (t)+ i(t)+ n(t) l=0 NS −1 ∑ (1.1) where sRF,l(t) is the useful GNSS signal received by the lth satellite in line of sight, i(t) is the additive interfering signal transmitted over a carrier frequency fint and characterized by a two-sided bandwidth Bint, and n(t) is the additive white Gaussian noise. The front-end block is in charge of demodulating the composite received signal to an intermediate frequency (IF) and passing it through a filter with bandwidth BIF to remove the image frequencies. At the output of the ADC/ AGC block of Figure 1.1, composed of the analog-to-digital converter (ADC) driven by the automatic gain control (AGC), the continuous signal is digi- tized in yIF(nTS), where Ts = 1/fs is the time sampling interval, and n is the discrete-time index. Thus, the composite received signal at the ADC/AGC output can be written as yIF [n] = yIF nTs ( ) = Qk u sIF,l nTs ( )+ iIF nTs ( )+ h nTs ( ) l=0 L−1 ∑ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ (1.2) where iIF(t) is the demodulated version of the interfering signal (filtered if Bint BIF) and η(t) is the filtered Gaussian noise, the function Qu k denotes the quantization over k bits, and Ts is the sampling interval. Expanding the term Figure 1.1 Functional blocks of GNSS receiver. 6519_Book.indb 25 12/19/14 3:37 PM
- 31. 26 GNSS InterferenceThreats and Countermeasures SIF,l(nTs), the expression for the single digitized GNSS signal affected by noise and interference components becomes (neglecting for the sake of simplicity the subscript l) yIF [n] = Qk u 2Cd n - n0 ⎡ ⎣ ⎤ ⎦c n - n0 ⎡ ⎣ ⎤ ⎦⋅ cos 2pFD,0 n + j0 ( )+ iIF [n]+ h[n] ⎡ ⎣ ⎤ ⎦ (1.3) where C is the received GNSS signal power from one satellite in view, d[n] and c[n] are, respectively, the navigation data message signal and the pseudo- random noise sequence, FD,0 = (fIF + f0)Ts is the Doppler-affected frequency, n0 = (τ0/Ts) is the digital code delay, ϕ0 is the instantaneous carrier phase, and i[n] and η[n] are the digitized interference and the digital Gaussian noise component, respectively. Given BIF, the front-end bandwidth, it can be shown that after sampling the signal at the Nyquist frequency fs = 2BIF, the noise variance becomes sIF 2 = E h2 [n] { } = N0 fs 2 = N0 BIF (1.4) where N0 /2 is the power spectral density (PSD) of the noise. In the acquisition block, Doppler frequency ˆ fd and code phase ˆ τ estimations are provided by correlations among the in-phase and quadrature components of the incoming signal and a GNSS code local replica. More details about the acquisition procedure are available, for example, in [24, 25] and are not addressed further in this chapter. The effect of the different types of interference on the acquisition stages is investigated in Chapter 2. The signal tracking follows the signal acquisition. Over each channel of the receiver, a delay lock loop (DLL) is used to synchronize the received spreading code and a local replica, while a phase lock loop (PLL) is generally employed to track the phase of the incoming carrier. The signal tracking relies on the properties of the signal correlation and is fundamental to demodulate the navigation message and estimate the range between the user and the sat- ellites. Conventional receiver architectures generally include a frequency lock loop (FLL) to refine the rough estimate performed by the signal acquisition. The FLL eases the PLL lock, reducing the transient time between the signal acquisition and the steady-state carrier/code tracking. Figure 1.2 shows the block diagram of a tracking system commonly used in digital GNSS receivers for a single channel, but the same architecture is repeated over all channels to track different satellites (or different channels from the same satellite in case of composite signals as foreseen for the Galileo system). 6519_Book.indb 26 12/19/14 3:37 PM
- 32. The Interference Threat27 Thetracking loop relies on correlation operations between the received signal and local replicas of carrier and code, initialized by the Doppler fre- quency ˆ fd and the code phase ˆ τ estimated in the acquisition phase. The values of correlation are then used to produce feedback control sig- nals on the basis of proper discrimination functions; one for the PLL and one for the DLL. Such control signals are filtered and used to steer the code and carrier generators that prepare the local replicas for the next loop iteration. The process continues and the system follows the input signal variations over time. Note that the described synchronization process corresponds to find- ing the best estimate of the local carrier frequency/phase and local code delay that maximize the correlation between the incoming and the local replicas. Noncoherent tracking systems, like that shown in Figure 1.2, use two branches, one in phase (I) and the other in quadrature (Q). Generally speaking, noncoherent tracking loops are more robust and do not require the estimate of the carrier phase (i.e., they do not necessarily need a PLL; an effective sys- tem can be designed combining an FLL and a DLL). For example, right after the signal acquisition, when the tracking phase starts, the system has not yet recovered the phase of the incoming carrier and part of the power goes on the quadrature branch. Different from coherent tracking loops (that use only the I branch), in this case, using both the branches, the discriminators are still able to produce feedback signals. If a PLL is used, after an initial transient time, the incoming carrier is synchronized with the local one and the received signal is completely converted on the I branch. Figure 1.2 Block diagram of a code and carrier tracking loop for GNSS receivers. 6519_Book.indb 27 12/19/14 3:37 PM
- 33. 28 GNSS InterferenceThreats and Countermeasures When both the DLL and PLL are locked, the incoming signal is despread and converted to baseband. The navigation data bits appear at the output of the in-phase prompt correlator and can be decoded. In addition, with the DLL locked, the local and the incoming codes are aligned. Referring to the local code, the receiver knows exactly when a new code period starts and is able to recognize navigation data bits and boundaries of the navigation message. The receiver stays synchronized to the tracked satellites, continuously counting the number of received chips, full code periods, navigation bits, and message frames. These counters are fundamental to measuring the misalignment over different channels and tracking different satellites, and are used to compute the pseudoranges. Once at least four of the pseudoranges are obtained the position is estimated by means of a trilateration procedure. In Chapter 2, the impact of the presence of i(t) on the different stages of the receiver is analyzed, showing the effect on the acquisition probabilities and on the tracking jitter. 1.5 Organization of the Book The book is divided into two parts. Chapters 1, 2, and 3 provide an overview and classification of interference and spoofing sources. The different sources are discussed in terms of their features (frequency, modulation, and so on) and their proper model with respect to the GNSS signals. Chapter 4 introduces some common techniques for the analytical assessment of the interference effects, and can be used as a reference for the prediction of the performance of a GNSS receiver in an interfered environment. The second part of the book is then devoted to describing the techniques for the detection and mitigation of interference and spoofing attacks. Chap- ter 5 presents an overview of the common interference detection techniques tailored to the different families of interference. Mitigation of interference is addressed by Chapters 6 and 7, presenting classical mitigation techniques and advanced signal processing techniques, respectively. Chapter 8 discusses the best strategies for providing antispoofing features to GNSS civil signals. References [1] http://www.gps.gov/spectrum/lightsquared/. [2] Humphreys T., et al., “Assessing the Spoofing Threat: Development of a Portable GPS Civilian Spoofer,” in Proc. of the 21st International Technical Meeting of the Satellite 6519_Book.indb 28 12/19/14 3:37 PM
- 34. The Interference Threat29 Divisionof the Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 2314–2325. [3] Yeh, K. C., and C.-H. Liu, “Radio Wave Scintillations in the Ionosphere,” Proc. IEEE, Vol. 70, No. 4, 1982, pp. 324–360. [4] Doherty, P. H., et al., “Ionospheric Scintillation Effects in the Equatorial and Auroral Regions,” Proc. 13th Int. Technical Meeting of the Satellite Division of the Institute of Navigation (ION GPS 2000), Salt Lake City, UT, pp. 662–671. [5] Betz, J. W., “Effect of Narrowband Interference on GPS Code Tracking Accuracy,” Proc. 2000 National Technical Meeting of the Institute of Navigation, Anaheim, CA, January 2000, pp. 16–27. [6] Titus, L. B. M., et al., “Intersystem and Intrasystem Interference Analysis Methodol- ogy,” in Proc. ION GPS/GNSS 2003, Portland, OR, September 2003. [7] Liu, W., et al., “GNSS RF Compatibility Assessment: Interference Among GPS, Gali- leo, and Compass,” GPS World, December 2010. [8] Mitch, R. H., et al., “Civilian GPS Jammer Signal Tracking and Geolocation,” Proc 25th Int. Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 2901–2920. [9] Borio, D., C. O’Driscoll, and J. Fortuny, “Jammer Impact on Galileo and GPS Receiv- ers,” Proc. 2013 Int. Conf. on Localization and GNSS (ICL-GNSS), June 25–27, 2013, pp. 1, 6. doi:10.1109/ICL-GNSS.2013.6577265 [10] Grant, A., and P. Williams, “GNSS Solutions: GPS Jamming and Linear Carrier Phase Combination,” Inside GNSS, Vol. 4, No. 1, January/February 2009. [11] Coffed, J., “The Threat of GPS Jamming. The risk to an Information Utility”; available at http://www.exelisinc.com/solutions/signalsentry/Documents/ThreatOfGPSJam- ming_February2014.pdf. [12] Jones S., and Hoyos C.,“GPS Pioneer Warns on Network’s Security,” Financial Times, http:// http://www.ft.com/cms/s/0/fadf1714-940d-11e3-bf0c-00144feab7de. html#axzz3J2VEueWr. [13] Gutierrez, P., “At ENC 2014: A GNSS Wake Up Call for Europe,” Inside GNSS News, April 16, 2014; available at http://www.insidegnss.com/node/3985. [14] Jewell, D., “Protect, Toughen, Augment: Words to the Wise from GPS Founder,” GPS World, April 15, 2014; available at http://gpsworld.com/protect-toughen- augment-words-to-the-wise-from-gps-founder. [15] Grabowsky, J. C., “Personal Privacy Jammers. Locating Jersey PPDs Jamming GBAS Safety-of-Life Signals,” GPS World, Vol. 23, No. 4, April 2012. [16] Pullen, S., and G. X. Gao, “GNSS Jamming in the Name of Privacy,” Inside GNSS, Vol. 7, No. 2, March/April 2012. [17] “LightSquared Fails FCC GPS Interference Tests,” 360 Degrees Column, Inside GNSS, Vol. 6, No. 4, July/August 2011, pp. 12–15. 6519_Book.indb 29 12/19/14 3:37 PM
- 35. 30 GNSS InterferenceThreats and Countermeasures [18] Boulton, R., et al., “GPS Interference Testing—Lab, Live, and LightSquared,” Inside GNSS, Vol. 6, No. 4, July/August 2011, pp. 32–45. [19] Grant, A., et al., “GPS Jamming and the Impact on Maritime Navigation,” Journal of Navigation, Vol. 62, No. 2, April 2009, pp 173–187. [20] Motella, B., M. Pini, and F. Dovis, “Investigation on the Effect of Strong Out-of-Band Signals on Global Navigation Satellite Systems Receivers,” GPS Solutions, Vol. 12, No. 2, March 2008, pp. 77–86. [21] De Bakker, P., et al., “Effect of Radio Frequency Interference on GNSS Receiver Output,” Proc. 3rd ESA Workshop on Satellite Navigation User Equipment Technologies (NAVITEC 2006), ESA/ESTEC, Noordwijk, The Netherlands, December 2006. [22] Balaei, A. T., B. Motella, and A. G. Dempster, “GPS Interference Detected in Sydney- Australia,” Proc. 2007 Int. Global Navigation Satellite System (IGNSS 2007) Conf., Sydney, Australia, December 2007. [23] Motella, B., et al., “Assessing GPS Robustness in Presence of Communication Sig- nals,” Communications Workshops 2009, June 14–18, 2009, pp. 1, 5. doi:10.1109/ ICCW.2009.5207985 [24] Kaplan, E., and C. Hegarty, Understanding GPS Principles and Applications, 2nd ed., Norwood, MA: Artech House, 2005. [25] Misra, P., and P. Enge, Global Positioning System: Signals, Measurements, and Perfor- mance, Lincoln, MA: Ganga-Jamuna Press, 2006. 6519_Book.indb 30 12/19/14 3:37 PM
- 36. 31 2 Classification of InterferingSources and Analysis of the Effects on GNSS Receivers Fabio Dovis, Luciano Musumeci, Beatrice Motella, and Emanuela Falletti 2.1 Introduction A global navigation satellite system (GNSS) receiver is vulnerable to several kinds of radio-frequency interference (RFI) due to the fact that it has to extract pseudorange information by processing the signal in space (SIS), which is received at a very low signal power. The nominal received power is on the order of magnitude of −160 dBW for all GNSSs, without taking into account extra attenuations that may be due to the local environment. Despite the weakness of the signals, the spread- spectrum nature of the SIS allows navigation receivers to recover timing information and to estimate the pseudoranges necessary to compute the user’s position by exploiting the gain obtained at the output of the correlation block. Even if the correlation process is theoretically able to mitigate the presence of nuisances in the bandwidth of interest, a real limitation can be the finite dynamic range of the receiver front-end. The presence of undesired RFI and 6519_Book.indb 31 12/19/14 3:37 PM
- 37. 32 GNSS InterferenceThreats and Countermeasures other channel impairments can result in degraded navigation accuracy or, in severe cases, in a complete loss of signal tracking. This chapter introduces a general classification of the interfering sources, presenting an overview of the main terrestrial systems that are potential sources of RFI for the GNSS signals. The second part of the chapter discusses their effect on the different stages of the GNSS receiver. 2.2 Classification of Interfering Source The classification of the main disturbances for GNSS receivers takes into account heterogeneous aspects. The emission types can be defined as being intentional (jamming) or unintentional as described in Chapter 1. The first are common for military scenarios even if jamming of civil applications starts to be common due to the availability of jamming devices on the market. Furthermore, a large number of communication systems present in our daily lives emit power that could interfere with the GNSS L-band, due to out-of-band emissions by these electronic systems. We turn now to a discussion of the classification of interfering sources, based on their spectral and time features. 2.2.1 Interference Spectral Features A general classification of the interfering signals is based on their spectral characteristics such as carrier frequency fint and bandwidth Bint, with respect to the GNSS signal carrier fGNSS and occupied bandwidth BGNSS • Out-of-band interference refers to interfering signals whose carrier frequency is located near to the targeted GNSS frequency band (fint fGNSS − BGNSS/2 or fint fGNSS + BGNSS/2. • In-band interference refers to interfering signals with carrier frequency within the GNSS frequency band ( fGNSS − BGNSS/2 fint fGNSS + BGNSS/2). Moreover, interference can be further classified according to its characteristics in the frequency domain as follows: • Narrowband interference (NBI): The spectral occupation is smaller with respect to the GNSS signal bandwidth (Bint ≪ BGNSS). 6519_Book.indb 32 12/19/14 3:37 PM
- 38. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers33 • Wideband interference (WBI): The spectral occupation is comparable with respect to the GNSS signal bandwidth (Bint ≈ BGNSS). • Continuous-wave interference (CWI): Represents the ultimate limit in NBI and appears as a single tone in the frequency domain (Bint → 0). Furthermore, in general, interference might have frequency-varying characteristics, for example, the chirp signals characterized by a linear varia- tion in time of the instantaneous frequency, thus appearing as WBI. This kind of interfering signal is typically generated by the jammers. Such devices are capable of transmitting strong power chirp signals sweeping several megahertz in a few microseconds, thus obscuring the correct signal recep- tion of each GNSS channel. Due to their availability on the web, this type of intentional interfering signal is gaining more and more attention in civil- ian applications. CWI could have a severe impact on a GNSS receiver, either on the acqui- sition or on the tracking process, because the interference power is dispersed on the whole search space by the correlation with the local code, compromising the acquisition accuracy and affecting the other functional blocks. The impact of CWI and NBI strongly depends on the value of the central frequency of the interference within the frequency band. This is due to the almost periodic nature of GNSS signals. In fact, the spectrum of a GNSS signal has compo- nents spaced at multiples of the inverse of the code period (e.g., 1 kHz for GPS C/A code) with different power allocated to each component depending on the shape of the code spectrum. The impact of CWI is larger in cases where the CWI is matched with such components [1–3]. 2.2.2 Pulsed Interference Pulsed interfering signals are characterized by an on–off status of short dura- tion (order of microseconds), which alternate in the time domain. This type of interference signal is typical of aviation scenarios, where several aeronautical radio navigation Services (ARNS) broadcast strong pulsed signals in a band- width that is shared with some of the satellite navigation systems. The parameters used to describe pulsed interference are: • Pulse width (PW): duration of one pulse; • Pulse repetition frequency (PRF): number of pulses per second; • Duty cycle (DC = PRF*PW): the percentage of time that is occupied by the pulses. 6519_Book.indb 33 12/19/14 3:37 PM
- 39. 34 GNSS InterferenceThreats and Countermeasures Pulsed interference with low DC has a small impact on receiver perfor- mance compared to continuous interference with the same power and center frequency. 2.3 Potential Interference Sources The potential interference can share the GNSS frequencies (in-band RFI) or be far from the GNSS carrier (out-of-band RFI). There are almost no in-band authorized emissions in the GNSS bandwidths; however, interference comes mainly from the spurious emissions of out-of-band systems, which generate harmonics that collide with the GNSS bandwidths. 2.3.1 Out-of-Band Signals In the following sections, some of the main potential out-of-band interference sources are analyzed. Analog TV Channels TV emissions are veritable sources of interference for a GNSS receiver. They can manifest as both wideband and narrowband interference: The video car- riers are considered to be medium/wideband signals, whereas the sound car- riers are considered to be CWI. In the broadcast TV signal, VHF and UHF bands are used. The harmonics of such bands generated by TV ground sta- tion transmitters can generate potentially dangerous interference for GNSS receivers as depicted in Figure 2.1. As an example, in [4] a case of interference from a TV signal is reported. In this case, the interference signal affects the active antenna LNA causing harmonic distortion in the same LNA that results in an average of 5-dB loss in C/N0. In [3] six TV channels, French and American equivalents, with their harmonics are analyzed in frequency and power terms. DVB-T Signals The DVB standard has been defined (since 1993) within an initiative involv- ing more than 300 European and extra-European members. The DVB proj- ect harmonized the strategies for introducing digital television and the new multimedia interactive services on transmission networks. It also defined the technical specifications. The project defined the system specifications for stan- dard Digital Video Broadcasting–Satellite (DVB-S), developed for the direct diffusion of TV multiprogramming from satellites and for standard Digital 6519_Book.indb 34 12/19/14 3:37 PM
- 40. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers35 Video Broadcasting–Cable (DVB-C) for the distribution of television signals through core networks. The DVB family also comprehends the standard for Digital Video Broadcasting–Terrestrial (DVB-T), for the provision of wire- less digital terrestrial television. The DVB-T standard is based on the Moving Pictures Experts Group-2 (MPEG-2) standard for audio/video signal source coding and it adopts a multi-carrier modulation COFDM to distribute the total data stream among a large number of carrier frequencies equally spaced and modulated using QPSK, 16-QAM, 64-QAM, nonuniform 16-QAM, or nonuniform 64-QAM [5]. Figure 2.1 Potential TV channel harmonic interference. 6519_Book.indb 35 12/19/14 3:37 PM
- 41. 36 GNSS InterferenceThreats and Countermeasures In the European broadcasting area, the DVB-T frequency bands are the VHF III (174–230 MHz), UHF IV (470–862 MHz), and UHF V (582–862 MHz) bands. These frequency values do not represent a direct threat to GNSS receivers, but they can cause some problems if harmonics due to potential distortions caused by the malfunction of some electronic devices, like power amplifiers, are considered. Even one single damaged amplifier in the amplifi- cation chain could cause nonlinear behavior, introducing spurious emissions at the RF output that, due to the high power level emitted, could represent a real threat for a nearby GNSS receiver. Furthermore, considering that the frequency involved in the DVB-T signal is the same of that of analog TV, the probability of having some disturbances caused by DVB-T signals can be considered similar to that of having spurious emissions from analog television systems. In [6–8] some examples of significant variations in the quality of the GPS signal due to analog television transmitters are reported. Considering, for example, the third harmonics1 of UHF V carrier, it would fall into the L1 GPS band representing a nonnegligible threat to the receiver. Therefore, it is important to evaluate the possibility of distortions caused by nonlinear amplifiers or linear ones in saturation. A detailed analysis of OFDM DVB-T potential interference in Europe is reported in [7] where the impact of RFI on the GNSS useful signal is evalu- ated by means of the spectral separation coefficient. VHFCOM Other VHF communication systems can be considered dangerous to a GNSS receiver [3, 9]. The VHF band (118–137 MHz) contains 760 channels spaced by 25 kHz, and it is commonly used by air traffic control (ATC) communica- tions. The harmonics are considered to be NBI with a bandwidth of about 25 kHz. The VHF channels, centered at 121.150, 121.175, and 121.200 MHz, have the 13th harmonic within the GPS bandwidth, whereas the channels centered at 131.200, 131.250, and 131.300 MHz are dangerous for their 12th harmonic. In Figure 2.2 VHF communication (VHFCOM) potential harmonics are depicted. FM Harmonics Also small frequency bands inside the FM band (87.5–108 MHz) have har- monics that fall in the GNSS bands. The channels at 104.9 and 105.1 MHz have their 15th harmonics near the GPS and Galileo bandwidths as depicted 1 The harmonic order is considered with respect to the signal fundamental frequency (f0) adopting the definition used in [8]. 6519_Book.indb 36 12/19/14 3:37 PM
- 42. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers37 in Figure 2.3. The channels are spaced at 150 kHz, while the maximum transmitted power is 50 dBW. The harmonics generated by FM sources are considered as WBI with respect to GNSS signals allocated in the L1/E1 bands. Personal Electronics Devices Personal electronic devices (PEDs) in proximity to a GNSS receiver can cause the disruption of GNSS signal reception. PEDs include cell phones, pagers, two-way radios, remote control toys, laptops, and many others. A larger num- ber of PEDs are expected to include, in the future, ultra-wideband (UWB) transmission that allows the development of high-bit-rate personal devices. SATCOM Satellite communications (SATCOM) operate in the frequency bands of 1626– 1660.5 MHz with channels spaced at 0.75 MHz and bandwidth of 20 kHz. Figure 2.2 Potential VHFCOM channel harmonic interference. 6519_Book.indb 37 12/19/14 3:37 PM
- 43. 38 GNSS InterferenceThreats and Countermeasures Multi-carrier transmission in a SATCOM service produces intermodulation products that can fall in the GNSS band. A possible example is reported in [3]. VOR and ILS Harmonics The VHF omnidirectional range (VOR) is a type of radio navigation system for aircraft that provides information about radial position with respect to ground station. The instrument landing system (ILS) consists of two radio transmitters providing lateral and vertical guidance to aircraft for approach- ing landing. VOR/ILS emitters are usually positioned at the beginning, end, and sides of airport runways. These approaching landing systems operate in the 108–117.95-MHz band including 200 channels frequency spaced at 50 kHz. In detail, the VOR uses 12 channels in the 112.24–112.816-MHz band while the ILS localizer transponder only uses one frequency on 40 channels in the 108.10- to 111.95-MHz band. Their harmonics, the 14th from VOR Figure 2.3 Potential FM harmonic interference. 6519_Book.indb 38 12/19/14 3:37 PM
- 44. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers39 and 2nd from ILS corresponding to 111.9 and 111.95 MHz, enter on the L1/ E1 band. They are considered CWI signals. Mobile Satellite Service (MSS) The mobile satellite service (MSS) system can generate two distinct inter- ference threats to a GNSS receiver. The MSS mobile earth stations use the 1610–1660.5-MHz band, potentially introducing wideband power in the GNSS band. Mobile Phone Interference In general, no direct consequences from mobile phones on a GNSS receiver have been reported in the literature so far. Some information is available for aircraft navigation equipment, where a GPS receiver is commonly used. In [10], an investigation of spurious emissions from six wireless phone technolo- gies is described, analyzing the effects on aircraft systems, among which is a GPS. The tests are carried out from semianechoic and reverberation cham- bers using wireless phone technologies with different transmission frequen- cies and different receiving antennas. The study is conducted evaluating the total radiated power from each cellular versus the frequencies of the system considered. In the analysis, the receiver sensitivity for the GPS receiver is −120 dBm, but a more realistic level is considered to be around −82 dBm. This value is obtained considering a minimum path loss of 38 dB. This gap is evaluated in [11], calculating the path loss after having generated signals inside the plane. The results show that all the considered values exceed the receiver system sensitivity level but at the same time are under the more real- istic value obtained from the path loss. So, the conclusions of the paper are that the radio-frequency emission from the phones tested do not interfere with the avionics system examined, among which is the GPS. 2.3.2 In-Band Signals Some interference sources broadcast signals whose carrier frequency is allocated in the GNSS bands, and thus they generate in-band interference. Chapter 1 discussed how intersystem and intrasystem interference have to be considered a primary source of in-band disturbance. However, the level of acceptable interference is defined during the design phase of the systems, and, so far, the acceptable level has been the result of international negotiations, discus- sions, and agreements (consider, e.g., the GPS/Galileo interoperability agree- ment). In this section the focus is on terrestrial non-GNSS systems. A short 6519_Book.indb 39 12/19/14 3:37 PM
- 45. 40 GNSS InterferenceThreats and Countermeasures description of the most relevant systems emitting power in one or more of the GNSS bands is provided in the following subsections. Military/Civil Aeronautical Communication Systems The military communications systems can be considered in-band interferers due to the signal band used by the systems. The Galileo E5a and E5b bands, located within 1164–1214 MHz, occupy frequencies already used for aero- nautical radio-navigation services (ARNS) such as for tactical air navigation (TACAN), distance measuring equipment (DME), and secondary surveillance radar (SSR), as well as by the DoD Joint Tactical Information Distribution System (JTIDS) and the Multifunction Information Distribution System (MIDS). Other aeronautical systems operate in these frequencies such as the Traffic Collision and Avoidance System (TCAS), Identification Friend or Foe (IFF), and planned Automatic Dependent Surveillance–Broadcast (ADS-B). The DME/TACAN systems consist of an airborne interrogator and a ground-based transponder that emits high-power pulsed signals that constitute a threat to GNSS receivers. DME and TACAN provide range measurements of the aircraft with respect to a ground reference station. The TACAN is a military system that provides range and azimuth measurements. The DME/ TACAN system operates in the 960- to 1215-MHz frequency band [12] in four different modes: X, Y, W, and Z, even if only the DME/TACAN ground transponder X mode occupies the 1151–1215 MHz frequency band that inter- feres with the E5a/L5 and E5b GNSS signal (see Figure 2.4 and Table 2.1). The analytical expression of the classical DME/TACAN pulse pair transmitted by the ground beacons is ypulse (t) = e − a 2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟t2 + e − a 2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟(t−∆t)2 (2.1) where, for example, for the X mode α = 4.5 ⋅ 1011 s−2 and the interpulse inter- val is Δt = 12 μs. The maximum pulse repetition frequency (PRF) for the DME and the TACAN system are 2700 and 3600 ppps, respectively. JTID/MIDS are spread-spectrum digital communications systems for exchanging data among military platforms. They operate between 969 and 1206 MHz interfering with the E5a/E5b bands as reported in Figure 2.5. Ultra-Wideband Signals The definition of UWB includes any signal that occupies more than 500 MHz between 3.1 and 10.6 GHz and meets the spectrum mask that defines the 6519_Book.indb 40 12/19/14 3:37 PM
- 46. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers41 Figure 2.4 Classical baseband DME/TACAN pulse pair. Figure 2.5 DME/TACAN and JTIDS/MIDS frequency plan. 6519_Book.indb 41 12/19/14 3:37 PM
- 47. 42 GNSS InterferenceThreats and Countermeasures indoor limits for UWB communication systems. UWB signals have emerged as a potential solution for low-complexity, low-cost, low-power consumption, and high-data-rate wireless connectivity. The technologies based on UWB offer simultaneous high-data-rate communication, with data transmission rates of 100 to 500 Mbps at distances of 2–10m using an average radiated power of a few hundred microwatts. UWB systems have also been studied for indoor location and navigation purposes because of their performance in multipath environments. The main advantages of UWB are the minimization of reflec- tion from clutter and the ability to penetrate structures with high data rates and high resolution, a low probability of interception by undesired receivers, and the possibility to be used for high-precision ranging. The data modulation schemes often utilized in UWB systems are pulse position modulation (PPM) and pulse amplitude modulation (PAM). The UWB signal is generated by using sub-nanosecond pulses that spread the signal energy on a wide frequency band. Thus, these systems employ low-power sig- nals but with an extremely wide bandwidth. This aspect is critical for systems such as GNSS whose signal power is far below the noise floor. Several studies Table 2.1 DME Operational Mode Classification Channel Mode Operating Mode Pulse Pair Spacing (μs) Time Delay (μs) Interrogation Reply First Pulse Timing Second Pulse Timing X DME/N DME/P IA M DME/P FA M 12 12 18 12 12 12 50 50 56 50 — — Y DME/N DME/P IA M DME/P FA M 36 36 42 30 30 30 56 56 62 50 — — W DME/N DME/P IA M DME/P FA M — 24 30 — 24 24 — 50 56 — — — Z DME/N DME/P IA M DME/P FA M — 21 27 — 15 15 — 56 62 — — — 6519_Book.indb 42 12/19/14 3:37 PM
- 48. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers43 [13–15], concluded that UWB signals can degrade GPS receiver performance. Other studies [16, 17] demonstrate by simulation and in a wireless personal area network (WPAN) case study, respectively, that UWB interference effect can be reduced by choosing the proper modulation parameters. 2.3.3 Classification of Jammers As explained in Chapter 1, the term jamming refers to intentional transmis- sion of RF interference with the goal of masking certain portions of frequency bands with noise. In the case of GNSS systems, a jammer (also referred to as a personal privacy device (PPD)) is able to jam (or block) GNSS signals, likely preventing the receivers from operating correctly within the jammer area. As an example, Figure 2.6 shows two different models of jammers. Both of them are able to transmit over different frequency bands, including the GNSS E1/ L1 band. Intentional interference is a well-known threat in military applications, but it is also considered a growing concern in the civil environment, thanks Figure 2.6 Examples of multifrequency GNSS jammers: adjustable desktop jammer (left) and four-antenna portable device (right). 6519_Book.indb 43 12/19/14 3:37 PM
- 49. 44 GNSS InterferenceThreats and Countermeasures to the fact that real incidents caused by PPDs have already occurred (consider the incident at the Newark Airport in New Jersey, described in Chapter 1). It is worth recalling that in many countries (e.g., United States or several European countries) jammers are illegal to sell or use. In spite of this, it might not be forbidden to own or buy a jammer, easily achievable through several websites, even at a cost of few tens of dollars [18]. Next, we summarize the main classifications of jammers proposed in the literature and discuss their main characteristics. A survey of jammers, specifically tailored to in-car jammers, is proposed in [19]. In-car jammers are small devices, powered by a car’s cigarette lighter. This class of jammers is particularly important, because their use might allow users (e.g., vehicles) to avoid being tracked. In [19] jammers are classified into four classes depending on their signal characteristics: A few of them transmit a continuous-wave (CW) signal, while the majority use a chirp signal. The signal bandwidth varies from less than 1 kHz (for the case of CW) to 44.9 MHz, with a sweep time in the interval [8.62 ÷ 18.97] μs. A further classification of jammers can be found in [20], where the categories are mainly based on power source. All the jammers analyzed in [20] are portable devices, divided into three groups: devices designed to plug into a car cigarette lighter’s 12-volt supply (Group 1), and devices powered by an internal rechargeable battery with (Group 2) or without (Group 3) an external antenna. As a consequence of the analysis of 18 different devices, the authors concluded that all of them use a swept tone method to generate broadband interference on the L1 or L2 band (with a sweep rate of 9 μs, on average, covering a bandwidth of 20 MHz). They also provided an estimate of the analyzed jammers’ effective ranges, which vary in the [300m ÷ 6 km] range for tracking, and the [600m ÷ 8.5 km] range for acquisition. A further survey of jammers can be found in [21], where multifrequency jammers, able to simultaneously disturb more than one GNSS band (L1, L2, and L5), are characterized. The analysis confirmed that cigarette lighter jam- mers only operate on the L1 band, with different values for the sweep period (with 9 μs being the most common value). It was also shown that the transmit- ted power can vary from −10 to more than 30 dBm and, in general, cigarette lighter jammers are characterized by lower power levels than multifrequency battery jammers. An example of a chirp signal generated by a portable jammer device is depicted in Figure 2.7. The figure shows the time-frequency representation of the signal emitted by the jammer. It can be seen that the chirp signal sweeps approximately 9 MHz during an interval of 10 μs. 6519_Book.indb 44 12/19/14 3:37 PM
- 50. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers45 2.4 The Impact of RFI on GNSS Receivers When subjected to very strong interference, a GNSS receiver can be totally blinded and stop working. This is often the scope of intentional jammers, who are attempting to deny the use of the GNSS-based positioning in a certain area or region. However, in a number of cases the presence of interference is severe enough to significantly decrease receiver performance, but not severe enough to make the receiver lose its lock on the satellite signals or blind the acquisition of the satellite signals. Such intermediate power values turn out to be the most dangerous cases, because sometimes they cannot be detected but they are leading to a worsening of the positioning performance. For the user of a GNSS receiver, the relevance of the effect of strong RFI is obvious. If the receiver is unable to track satellites, it cannot calculate its position. When the receiver is able to track satellites, but the signal is affected by RFI, the error on the pseudorange measurements is increased. The accuracy of the position solution depends, among others, on the quality of the pseudorange measurements and/or the phase measurements. As a consequence, when RFI degrades the pseudorange and phase measurements or induces cycle slips on the phase measurements, the accuracy of the position solution will decrease. Figure 2.7 Example of chirp signal transmitted by a portable jammer: time–frequency representation. 6519_Book.indb 45 12/19/14 3:37 PM
- 51. 46 GNSS InterferenceThreats and Countermeasures In the following sections the effects on the different stages of the receiver are discussed. 2.4.1 Impact on the Front-End The front-end of the receiver is the first stage of the receiver affected by the presence of an interfering source. The front-end filters the incoming signal in the bandwidth of interest, demodulating it to the chosen intermediate fre- quency before performing the analog-to-digital conversion (ADC). We must consider the presence in the front-end of the adjustable gain control (AGC) between the analog portion of the front-end and the ADC. The variable gain amplifier adjusts the power of the incoming signal to optimize the signal dynamics for the ADC in order to minimize quantization losses. Nowadays, in fact, all modern receivers are designed as multibit equipment, thus requiring the presence of an AGC. When the GNSS band is interference free, which should be the norm due to restrictions on emissions in and near the band, AGC gain depends almost exclusively on thermal noise, since the received GNSS signal power level is below that of the thermal noise floor. The primary role of the AGC is to adjust the signal dynamics for variations in the received power due to the elevation of the satellite and/or different active antenna gain values. The statistics for samples at the ADC output in the case of an inter- ference-free GNSS band, reported in Figure 2.8(a), are basically normally distributed, as shown in Figure 2.8(b). When in-band interference is present, the AGC will squeeze the incoming signal in order to match the maximum dynamics of the ADC, thus causing a reduction of the amplitude of the useful signal, which may be lost. This is typical of what may happen in the presence of some kind of WBI; that being spread over a bandwidth larger than the passband of the front-end filter can be seen as additional noise on the bandwidth of interest. Furthermore, in the presence of NBI or CWI, the statistics of the digital signal at the output of the ADC are also affected. This can be seen in Figure 2.8(d), where the boundary quantization levels become more probable than the others. In this case the AGC is still able to compress the input signal to avoid a stronger saturation, however, the following stages of the receiver will have to deal with a GNSS contribution that is quantized only on the lower levels. In the presence of stronger interference, even the other components of the front-end (filters and amplifiers) may be led to work outside of their nominal regions, generating nonlinear effects or clipping phenomena (in which the signal amplitude exceeds the hardware’s capability to treat them). In both 6519_Book.indb 46 12/19/14 3:37 PM
- 52. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers47 Figure 2.8 (a) GPS L1 C/A code PSD in the absence of interference. (b) Histograms of the samples at the ADC output in the absence of interference. cases spurious harmonics are generated and mixed with the useful signal in the front-end itself. 2.4.2 Impact on the Acquisition Stage If the interference is not driving the AGC/ADC to full saturation, the acquisi- tion module is still able to perform its task, processing the interfered signal to estimate the code phase and the Doppler shift with respect to the local code. 6519_Book.indb 47 12/19/14 3:37 PM
- 53. 48 GNSS InterferenceThreats and Countermeasures Figure 2.8 (Continued) (c) GPS L1 C/A code PSD in the presence of CWI. (d) Histograms of the samples at the ADC output in presence of CWI. The correlation with the local code can be seen as a spreading operation fol- lowed by a filter. In [22] an exhaustive derivation of the impact of CWI on the acquisi- tion stage of a GNSS receiver is provided. In the presence of CWI the expres- sion of the digitized signal2 at the input of the baseband processing block of a GNSS receiver is 2 For the sake of simplicity, in this description the effect of the data and of the quantization effects are neglected. 6519_Book.indb 48 12/19/14 3:37 PM
- 54. Classification of InterferingSources and Analysis of the Effects on GNSS Receivers49 yIF [n] = 2Cc n - t0 ⎡ ⎣ ⎤ ⎦cos 2p fIF + fD,0 ( )Ts n + j0 ( ) +Aint cos 2pFint Ts n + qint ( )+ h[n] (2.2) where the first member of the sum is the useful received GNSS signal; Aint, Fint, and θint are, respectively, the amplitude, the carrier frequency, and the random phase uniformly distributed in the range [−π ;π) of the CWI assumed to be a pure sinusoid; and WIF[n] is the Gaussian noise component that, under the Nyquist sampling theorem assumption, can be assumed to be a classical independent and identically distributed (IID) discrete random process. According to the equivalent scheme of a GNSS acquisition block depicted in Figure 2.9, the signal in (2.2) is first multiplied by a complex exponential and then multiplied and integrated with respect to the local code chosen according to an hypothesis of Doppler frequency fD and code delay τ, thus providing the following cross ambiguity function complex components: SI t, fD ( ) = 1 N rI [n]c[n − t] = rI [t]∗ hc [t] n=0 N−1 ∑ SQ t, fD ( ) = 1 N rQ [n]c[n − t] = rQ [t]∗ hc [t] n=0 N−1 ∑ (2.3) where hc is the equivalent filter representing the operation of multiplication and integration with respect the local code, and N is the number of coherent integrated samples. Finally the CAF is obtained as a complex modulus Figure 2.9 Equivalent scheme of a GNSS acquisition block. 6519_Book.indb 49 12/19/14 3:37 PM
- 55. Exploring the Varietyof Random Documents with Different Content
- 56. 384. Fr. Guicciardini,l. XIX, p. 501. — Mém. de M. du Bellay, l. III, p. 100. — P. Jovii, l. XXVI, p. 52. — Bern. Segni, l. II, p. 43. — Mém. de Blaise de Montluc, l. I, p. 71, t. XXII. 385. Mém. de M. du Bellay, l. III, p. 106. — Fr. Guicciardini, l. XIX, p. 502. — Ben. Varchi, l. VI, p. 155. 386. P. Jovii, l. XXVI, p. 53. — Bern. Segni, l. II, p. 42. 387. Fr. Guicciardini, l. XIX, p. 502. — Mém. de M. du Bellay, l. III, p. 107. — Ben. Varchi, l. VI, p. 156. — P. Jovii, l. XXVI, p. 55. — Fr. Belcarii, l. XX, p. 618. 388. Fr. Guicciardini, l. XIX, p. 503 — P. Jovii Hist., l. XXVI, p. 56. — Mém. de M. du Bellay, l. III, p. 108. 389. P. Jovii, l. XXVI, p. 57, 58. — Fr. Guicciardini, l. XIX, p. 504. — Bern. Segni, l. II, p. 45. — Georg. von Frundsberg, B. VIII, f. 161. 390. P. Jovii Hist., l. XXVI, p. 59. — Bern. Segni, l. II, p. 44. — Arn. Ferroni, l. VIII, p. 170. 391. Fr. Guicciardini, l. XIX, p. 504. — M. du Bellay, l. III, p. 109. — Ben. Varchi, l. VI, p. 157. — Fr. Belcarii, l. XX, p. 619. 392. P. Jovii Hist., l. XXVI, p. 61. 393. B. Varchi, l. VI, p. 159. — Ber. Segni, l. II, p. 45. 394. P. Jovii Hist. sui temporis, l. XXVI, p. 61. — Ben. Varchi, l. VI, p. 158. — Alfonso de Ulloa vita di Carlo V, l. II, f. 115. 395. P. Jovii, l. XXVI, p. 61. — B. Varchi, l. VI, p. 158. — Alf. de Ulloa vita di Carlo V, l. II, f. 115. 396. Fr. Guicciardini, l. XIX, p. 511. — P. Jovii, l. XXVI, p. 77. — Marco Guazzo, f. 62. — P. Paruta, l. VI, p. 450. 397. Fr. Guicciardini, l. XIX, p. 506. — P. Jovii Hist., l. XXVI, p. 71. — Mém. de M. du Bellay, l. III, p. 112. — Ben. Varchi, l. VII, p. 170. — Bern. Segni, l. II, p. 47. — Ag. Giustiniani, l. VI, f. 282. — Qui finisce questa cronica genovese contemporanea. — P. Folieta, l. XII, p. 735. 398. Fr. Guicciardini, l. XIX, p. 508. — P. Jovii, l. XXVI, p. 72. — Mém. de Martin du Bellay, l. III, p. 114. — B. Varchi, l. VII, p. 178. — Fr. Belcarii, l. XX, p. 620. — Gal. Capella, l. VIII, f. 87. — P. Paruta, l. VI, p. 451. — Lett. de' Princ., t. II, f. 133. — Arn. Ferroni, l. VIII, p. 170. — B. Segni, l. II, p. 47. — P. Bizarri, l. XX, p. 475. — P. Folietae, Cont. Hist. Gen. Uberti ejus fratris, l. XII, p. 742. — Qui finisce questa storia. 399. Ben. Varchi Stor. Fior., l. VII. p. 173.
- 57. 400. Ivi, p.174. 401. Ivi, p. 175. 402. Il senatore Battista Lomellini lo ringraziò a nome dalla patria, e la repubblica gli fece innalzare una statua di marmo con questa iscrizione. «Andreæ Auriæ civ. opt. felicissimoque, vindici atque auctori publicae libertatis S. P. q. G. posuere.» Bern. Segni, l. II, p. 47 — P. Bizzarri, l. XX, p. 476. 403. Pet. Bizarri Sentinatis dissert. de Repub. Gen. statu, et administ. in Graevii Thesaur., t. I, p. II, p. 1453. 404. I nomi di questi ventotto alberghi furono, Auria (Doria), Calvi, Catani, Centurioni, Cibo, Cicada, Fieschi, Franchi, Fornari, Gentili, Grimaldi, Grilli, Giustiniani, Imperiali, Interiani, Lercari, Lomellini, Marini, Negri, Negroni, Palavicini, Pinelli, Promontori, Spinola, Salvaghi, Sauli, Vivaldi, Ususmari. 405. Fr. Guicciardini, l. XIX, p. 508. — Ben. Varchi, l. VII, p. 180. 406. La legge viene riportata da Grevio. Thes. Rer. Ital., t. I, p. II, p. 1471. 407. Hier. de Marinis de Reip. Genuens. Gubernat., c. II, in Graevi Thes., t. I, p. II, p. 1422 circa il 1667. 408. Ben. Varchi Stor. Fior., l. VII, p. 181. — Pet. Bizarri, dissert. de Reip. Genuens, adm. Thesaur. Ital., t. I, p. II, p. 1453 e seguenti. — Cont. Uberti Folietae a Paulo Fratre, l. XII, p. 741. — Jac. Bonfadii An. Genuens., l. I, p. 1341, in Graev. Thesauro, t. I, p. II. — Filippo Casoni Annali di Genova, t. II, l. III, p. 45 e segu. 409. La legge permetteva al senato d'ammettere ogni anno sette abitanti della città, e tre delle riviere nel corpo della nobiltà; purchè la di lui scelta cadesse sopra coloro che per natali, per costumi e per servigj renduti allo stato potevano di già essere riputati eguali ai nobili. Fil. Casoni Ann. di Genova, t. II, l. III, p. 46. 410. Gio. Cambi Hist. Fior., t. XXIII, p. 1. 411. Jac. Nardi Ist. Fior., l. VIII, p. 336. 412. Ben. Varchi, l. III, p. 170-176. — Ber. Segni, l. I, p. 14, 29. — Fil. de' Nerli, l. VIII, p. 162. 413. Ivi, p. 177. 414. Bern. Segni Ist. Fior., l. I, p. 19. 415. Ben. Varchi, l. VII, t. II, p. 203-215. Bern. Segni, l. I, p. 19. Questi porta la mortalità a 250,000 persone in tutto lo stato fiorentino. 416. Ben. Varchi, l. VII, p. 212.
- 58. 417. Jac. NardiIst. Fior., l. VIII, p. 339. — Comment. di Filippo de' Nerli, l. VII, p. 168. 418. Ben. Varchi, t. II, l. V, p. 53. — Jac. Nardi, l. VIII, p. 340. — Filip. de' Nerli, l. VIII, p. 170. — Bern. Segni, l. I, p. 31. — Gio. Cambi, t. XXIII, p. 5. 419. Ben. Varchi, l. VI, p. 133. — Bern. Segni, l. I, p. 31. — Filippo de' Nerli, l. VIII, p. 171. 420. Ben. Varchi, l. IV, t. I, p. 191. — Jacopo Nardi, l. VIII, p. 337. — Bern. Segni, l. I, p. 25. 421. Comment. di Filippo de' Nerli, l. VIII, p. 165. 422. Jacopo Nardi, l. VIII, p. 335. — Ben. Varchi, l. VII, t. II, p. 188. 423. Ben. Varchi, l. VII, p. 190. — Bern. Segni, l. II, p. 36. 424. Jacopo Nardi Ist. Fior., l. VIII, p. 337, 338. 425. Bern. Segni, l. I, p. 14 — Ben. Varchi, l. III, p. 150 e l. V, p. II. — Jac. Nardi, l. VIII, p. 341. 426. Bern. Segni Ist. Fior., l. II, p. 52-56. 427. Bern. Segni, l. II, p. 48. — P. Jovii Hist. sui temp., l. XXVI, p. 79. — Jac. Bonfacii An. Gen., l. I, p. 1344. — Galeat. Capella, l. VIII, p. 689. 428. Ben. Varchi, l. VIII, p. 287. 429. Gal. Capella, l. VIII, f. 89. 430. P. Jovii Hist. sui temp., l. XXVI, p. 81. — Gal. Capella, l. VIII, f. 90. 431. P. Jovii Hist., l. XXVI, p. 82. — Fr. Guicciardini, l. XIX, p. 521. — Gal. Capella, l. VII, f. 91. — Mém. de M. du Bellay, l. III, p. 117-121. — B. Segni, l. III, p. 74. — Jac. Nardi, l. VIII, p. 348. — Ben. Varchi, l. VIII, p. 289. — Fr. Belcarii, l. XX, p. 625. — P. Paruta, l. VI, p. 481. 432. Mém. de Martin du Bellay l. III, p. 122. — Ben. Varchi, l. IX, t. III, p. 6. — Fr. Guicciardini, l. XIX, p. 524. — Jac. Nardi, l. VIII, p. 347. — Fr. Belcarii, l. XX, p. 626. 433. Ben. Varchi, l. VIII, t. II, p. 224; l. IX, t. III, p. 4 e 5. 434. Fr. Guicciardini, l. XIX, p. 519. — B. Varchi, l. IX, p. 4. — P. Paruta, l. VI, p. 486. 435. Lett. de' Principi, t. II, f. 151. 436. P. Paruta, l. VI, p. 456. — Lettere dei Princ., t. II, f. 165, e frequentemente altrove. — Lettera del papa a Francesco I del 9 luglio 1528, f. 105.
- 59. 437. Lett. de'Princ., t. II passim e specialmente a f. 184. 438. Lett. de Princ., t. II, f. 167. 439. Risposta data a M. di Longavalle a nome di papa Clemente. Lett. de' Princ., t. II, f. 85. 440. Ben. Varchi, l. VIII, p. 291. — P. Jovii, l. XXVII, p. 84. — Bernardo Segni, l. III, p. 70. — Lettere de' Princ., t. II, f. 178, relative alla missione dell'arcivescovo di Capoa. 441. Ben. Varchi, l. VIII, p. 219. — Filippo de' Nerli, l. VIII, p. 169. — Ber. Segni, l. II, p. 49. — Lettera di Gio. Battista Sanga a Baldassare Castiglione, nunzio in Ispagna, del 10 febbrajo 1529, t. II, Lettere de' Principi, f. 154. 442. Fr. Guicciardini, l. XIX, p. 521. — P. Jovii Hist. sui temp., l. XXVII, p. 85. — Ben. Varchi, l. VIII, p. 292-294. — Bern. Segni, l. III, p. 71. — Jac. Nardi, l. VIII, p. 342-347. 443. Ben. Varchi Stor. Fior., l. IX, p. 10. — Rymer Acta pub., t. XIV, p. 335 e 340. 444. B. Varchi Stor. Fior., t. III, l. IX, p. 11. — Fr. Guicciardini, l. XIX, p. 523. — Bern. Segni, l. III, p. 73. — Filippo de' Nerli, l. VIII, p. 183. — Jac. Nardi Ist. Fior., l. VIII, p. 346. — P. Paruta, l. VI, p. 491. — Rymer Acta, t. XIV, p. 336. 445. Hist. de la Diplomatie française, l. III, p. 358. 446. Ben. Varchi, l. IX, p. 11. 447. Hist. de la diplom. fran., l. III, p. 355-359. — Mém. de M. du Bellay, l. III, p. 122. — Ben. Varchi, l. IX, p. 8. — P. Paruta, l. VI, p. 492. — Ar. Ferroni, l. VIII, p. 174 — Gal. Capella, l. VIII, f. 93. — Il trattato trovasi per disteso in Rymer Acta pub., t. XIV, p. 326-344. 448. Fr. Guicciardini, l. XIX, p. 524. — Ben. Varchi, l. IX, p. 4. 449. Fr. Guicciardini, l. XIX, p. 525. — Benedetto Varchi, l. IX, p. 14. — Filip. dei Nerli, l. IX, p. 185. 450. Ben. Varchi Stor. Fior. l. IX, p. 23. — Jac. Bonfadii An. Genuens., l. II, p. 1349. — Bern. Segni Stor. Fior., l. III, p. 76. 451. Fr. Guicciardini, l. XIX, p. 526. — P. Jovii, l. XXVII, p. 93. — Jac. Bonfadii, l. II, p. 1349. — Fr. Belcarii, l. XX, p. 627. 452. Fr. Guicciardini, l. XIX, p. 525. — Ben. Varchi, l. IX, p. 24. — P. Bizarri, l. XX, p. 479. — P. Paruta, l. VI, p. 489. — Lettere de' Princ., t. II, f. 160. 453. Fr. Guicciardini, l. XIX, p. 527. — Ben. Varchi, t. III, l. IX, p. 14. 454. Ben. Varchi, l. X, p. 235. — P. Jovii Hist. sui temp., l. XXVII, p. 92. — Alf. de Ulloa vita di Carlo V, l. II, f. 117. — Jo. Sleidani Comm. de statu Relig. et
- 60. Reip., l. VI,f. 102. 455. Istruzione al vescovo di Vaison, nunzio presso l'imperatore, intorno al modo da tenersi da questi verso gli stati italiani. Roma; 25 agosto 1529. Lett. de' Princ., t. II, f. 181. 456. Fr. Guicciardini, l. XIX, p. 528. — Jac. Nardi, l. VIII, p. 348. — Bern. Segni, l. III, p. 75. — P. Jovii, l. XXVII, p. 95. 457. Fr. Guicciardini, l. XIX, p. 529. — B. Segni, l. III, p. 90. — P. Jovii, l. XXVII, p. 96. — P. Paruta, l. VI, p. 490. — Gal. Capella, l. VIII, p. 94. 458. Ben. Varchi, l. X, p. 202. — P. Jovii, l. XXVII, p. 100. Lettera del papa all'imperatore, da Bologna il 27 ottobre. Lett. de' Princ., t. II, f. 186. 459. Fr. Guicciardini, l. XIX, p. 536. — Ben. Varchi, l. X, p. 252. — Bern. Segni, l. III, p. 92. — P. Jovii, l. XXVII, p. 100. — Fr. Belcarii, l. XX, p. 628. — Galeat. Capella, l. VIII, p. 94. — P. Paruta, l. VI, p. 495 — P. Giovio vita d'Alf. d'Este, p. 132. 460. Ben. Varchi, l. X, p. 251. 461. Fr. Guicciardini, l. XIX, p. 537. — Ben. Varchi, l. X, p. 256. — Bern. Segni, l. III, p. 94. — P. Jovii, l. XXVII, p. 103. — Gal. Capella, l. VIII, f. 94. — P. Paruta, l. VI, p. 500. 462. Fr. Guicciardini, l. XIX, p. 537. — Bern. Segni, l. III, p. 94. — Gal. Capella, l. VIII, f. 96 ed ultimo. 463. P. Paruta, l. VII, p. 559. — Mém. de M. du Bellay, l. VI, p. 300. — Murat. Ann. ad annum. 464. P. Paruta Ist. Ven., l. VI, p. 505. — Fr. Guicciardini, l. XIX, p. 538. — Ben. Varchi, l. X, p. 257. — P. Jovii Hist., l. XXVII, p. 104. 465. Ben. Varchi, t. IV, l. XI, p. 58. — Muratori Ann. d'Italia ad an. — P. Giovio vita d'Alfonso d'Este, p. 134. 466. Fr. Guicciardini, l. XX, p. 549. — Ben. Varchi, t. IV, l. XII, p. 349. — Muratori Ann. d'Italia, t. X, p. 242. — P. Jovii Hist., l. XXVII, p. 137. — Lo stesso, vita d'Alfonso d'Este, p. 137. 467. P. Jovii Hist. sui temp., l. XXVII, p. 110. 468. Ben. Varchi, l. XI, p. 59. — P. Jovii, l. XXVII, p. 110. 469. P. Jovii Hist. sui temp., l. XXVII, p. 110. — Mém. de M. du Bellay, l. IV, p. 140. 470. Fr. Guicciardini, l. XX, p 541. — P. Jovii Hist., l. XXVII, p. 105. — Bern. Segni, l. IV, p. 107. — Ist. di Gio. Cambi, t. XXIII, p. 51. — P. Paruta, l. VII, p. 510. — Alfonso de Ulloa vita di Carlo V, l. II, f. 119.
- 61. 471. Ben. VarchiStor. Fior., t. III, l. IX, p. 8, t. IV, l. XI, p. 60. — Bern. Segni, l. IV, p. 115. — P. Bizarri, l. XX, p. 489. — Alfonso de Ulloa vita di Carlo V, l. III, f. 121. — P. Paruta Ist. Ven., l. VII, p. 511.
- 62. Nota del Trascrittore Ortografiae punteggiatura originali sono state mantenute, correggendo senza annotazione minimi errori tipografici.
- 63. *** END OFTHE PROJECT GUTENBERG EBOOK STORIA DELLE REPUBBLICHE ITALIANE DEI SECOLI DI MEZZO, V. 15 (OF 16) *** Updated editions will replace the previous one—the old editions will be renamed. Creating the works from print editions not protected by U.S. copyright law means that no one owns a United States copyright in these works, so the Foundation (and you!) can copy and distribute it in the United States without permission and without paying copyright royalties. Special rules, set forth in the General Terms of Use part of this license, apply to copying and distributing Project Gutenberg™ electronic works to protect the PROJECT GUTENBERG™ concept and trademark. Project Gutenberg is a registered trademark, and may not be used if you charge for an eBook, except by following the terms of the trademark license, including paying royalties for use of the Project Gutenberg trademark. If you do not charge anything for copies of this eBook, complying with the trademark license is very easy. You may use this eBook for nearly any purpose such as creation of derivative works, reports, performances and research. Project Gutenberg eBooks may be modified and printed and given away—you may do practically ANYTHING in the United States with eBooks not protected by U.S. copyright law. Redistribution is subject to the trademark license, especially commercial redistribution. START: FULL LICENSE
- 64. THE FULL PROJECTGUTENBERG LICENSE
- 65. PLEASE READ THISBEFORE YOU DISTRIBUTE OR USE THIS WORK To protect the Project Gutenberg™ mission of promoting the free distribution of electronic works, by using or distributing this work (or any other work associated in any way with the phrase “Project Gutenberg”), you agree to comply with all the terms of the Full Project Gutenberg™ License available with this file or online at www.gutenberg.org/license. Section 1. General Terms of Use and Redistributing Project Gutenberg™ electronic works 1.A. By reading or using any part of this Project Gutenberg™ electronic work, you indicate that you have read, understand, agree to and accept all the terms of this license and intellectual property (trademark/copyright) agreement. If you do not agree to abide by all the terms of this agreement, you must cease using and return or destroy all copies of Project Gutenberg™ electronic works in your possession. If you paid a fee for obtaining a copy of or access to a Project Gutenberg™ electronic work and you do not agree to be bound by the terms of this agreement, you may obtain a refund from the person or entity to whom you paid the fee as set forth in paragraph 1.E.8. 1.B. “Project Gutenberg” is a registered trademark. It may only be used on or associated in any way with an electronic work by people who agree to be bound by the terms of this agreement. There are a few things that you can do with most Project Gutenberg™ electronic works even without complying with the full terms of this agreement. See paragraph 1.C below. There are a lot of things you can do with Project Gutenberg™ electronic works if you follow the terms of this agreement and help preserve free future access to Project Gutenberg™ electronic works. See paragraph 1.E below.
- 66. 1.C. The ProjectGutenberg Literary Archive Foundation (“the Foundation” or PGLAF), owns a compilation copyright in the collection of Project Gutenberg™ electronic works. Nearly all the individual works in the collection are in the public domain in the United States. If an individual work is unprotected by copyright law in the United States and you are located in the United States, we do not claim a right to prevent you from copying, distributing, performing, displaying or creating derivative works based on the work as long as all references to Project Gutenberg are removed. Of course, we hope that you will support the Project Gutenberg™ mission of promoting free access to electronic works by freely sharing Project Gutenberg™ works in compliance with the terms of this agreement for keeping the Project Gutenberg™ name associated with the work. You can easily comply with the terms of this agreement by keeping this work in the same format with its attached full Project Gutenberg™ License when you share it without charge with others. 1.D. The copyright laws of the place where you are located also govern what you can do with this work. Copyright laws in most countries are in a constant state of change. If you are outside the United States, check the laws of your country in addition to the terms of this agreement before downloading, copying, displaying, performing, distributing or creating derivative works based on this work or any other Project Gutenberg™ work. The Foundation makes no representations concerning the copyright status of any work in any country other than the United States. 1.E. Unless you have removed all references to Project Gutenberg: 1.E.1. The following sentence, with active links to, or other immediate access to, the full Project Gutenberg™ License must appear prominently whenever any copy of a Project Gutenberg™ work (any work on which the phrase “Project Gutenberg” appears, or with which the phrase “Project Gutenberg” is associated) is accessed, displayed, performed, viewed, copied or distributed:
- 67. This eBook isfor the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook. 1.E.2. If an individual Project Gutenberg™ electronic work is derived from texts not protected by U.S. copyright law (does not contain a notice indicating that it is posted with permission of the copyright holder), the work can be copied and distributed to anyone in the United States without paying any fees or charges. If you are redistributing or providing access to a work with the phrase “Project Gutenberg” associated with or appearing on the work, you must comply either with the requirements of paragraphs 1.E.1 through 1.E.7 or obtain permission for the use of the work and the Project Gutenberg™ trademark as set forth in paragraphs 1.E.8 or 1.E.9. 1.E.3. If an individual Project Gutenberg™ electronic work is posted with the permission of the copyright holder, your use and distribution must comply with both paragraphs 1.E.1 through 1.E.7 and any additional terms imposed by the copyright holder. Additional terms will be linked to the Project Gutenberg™ License for all works posted with the permission of the copyright holder found at the beginning of this work. 1.E.4. Do not unlink or detach or remove the full Project Gutenberg™ License terms from this work, or any files containing a part of this work or any other work associated with Project Gutenberg™. 1.E.5. Do not copy, display, perform, distribute or redistribute this electronic work, or any part of this electronic work, without prominently displaying the sentence set forth in paragraph 1.E.1
- 68. with active linksor immediate access to the full terms of the Project Gutenberg™ License. 1.E.6. You may convert to and distribute this work in any binary, compressed, marked up, nonproprietary or proprietary form, including any word processing or hypertext form. However, if you provide access to or distribute copies of a Project Gutenberg™ work in a format other than “Plain Vanilla ASCII” or other format used in the official version posted on the official Project Gutenberg™ website (www.gutenberg.org), you must, at no additional cost, fee or expense to the user, provide a copy, a means of exporting a copy, or a means of obtaining a copy upon request, of the work in its original “Plain Vanilla ASCII” or other form. Any alternate format must include the full Project Gutenberg™ License as specified in paragraph 1.E.1. 1.E.7. Do not charge a fee for access to, viewing, displaying, performing, copying or distributing any Project Gutenberg™ works unless you comply with paragraph 1.E.8 or 1.E.9. 1.E.8. You may charge a reasonable fee for copies of or providing access to or distributing Project Gutenberg™ electronic works provided that: • You pay a royalty fee of 20% of the gross profits you derive from the use of Project Gutenberg™ works calculated using the method you already use to calculate your applicable taxes. The fee is owed to the owner of the Project Gutenberg™ trademark, but he has agreed to donate royalties under this paragraph to the Project Gutenberg Literary Archive Foundation. Royalty payments must be paid within 60 days following each date on which you prepare (or are legally required to prepare) your periodic tax returns. Royalty payments should be clearly marked as such and sent to the Project Gutenberg Literary Archive Foundation at the address specified in Section 4, “Information
- 69. about donations tothe Project Gutenberg Literary Archive Foundation.” • You provide a full refund of any money paid by a user who notifies you in writing (or by e-mail) within 30 days of receipt that s/he does not agree to the terms of the full Project Gutenberg™ License. You must require such a user to return or destroy all copies of the works possessed in a physical medium and discontinue all use of and all access to other copies of Project Gutenberg™ works. • You provide, in accordance with paragraph 1.F.3, a full refund of any money paid for a work or a replacement copy, if a defect in the electronic work is discovered and reported to you within 90 days of receipt of the work. • You comply with all other terms of this agreement for free distribution of Project Gutenberg™ works. 1.E.9. If you wish to charge a fee or distribute a Project Gutenberg™ electronic work or group of works on different terms than are set forth in this agreement, you must obtain permission in writing from the Project Gutenberg Literary Archive Foundation, the manager of the Project Gutenberg™ trademark. Contact the Foundation as set forth in Section 3 below. 1.F. 1.F.1. Project Gutenberg volunteers and employees expend considerable effort to identify, do copyright research on, transcribe and proofread works not protected by U.S. copyright law in creating the Project Gutenberg™ collection. Despite these efforts, Project Gutenberg™ electronic works, and the medium on which they may be stored, may contain “Defects,” such as, but not limited to, incomplete, inaccurate or corrupt data, transcription errors, a copyright or other intellectual property infringement, a defective or
- 70. damaged disk orother medium, a computer virus, or computer codes that damage or cannot be read by your equipment. 1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the “Right of Replacement or Refund” described in paragraph 1.F.3, the Project Gutenberg Literary Archive Foundation, the owner of the Project Gutenberg™ trademark, and any other party distributing a Project Gutenberg™ electronic work under this agreement, disclaim all liability to you for damages, costs and expenses, including legal fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH DAMAGE. 1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a defect in this electronic work within 90 days of receiving it, you can receive a refund of the money (if any) you paid for it by sending a written explanation to the person you received the work from. If you received the work on a physical medium, you must return the medium with your written explanation. The person or entity that provided you with the defective work may elect to provide a replacement copy in lieu of a refund. If you received the work electronically, the person or entity providing it to you may choose to give you a second opportunity to receive the work electronically in lieu of a refund. If the second copy is also defective, you may demand a refund in writing without further opportunities to fix the problem. 1.F.4. Except for the limited right of replacement or refund set forth in paragraph 1.F.3, this work is provided to you ‘AS-IS’, WITH NO OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED,
- 71. INCLUDING BUT NOTLIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. 1.F.5. Some states do not allow disclaimers of certain implied warranties or the exclusion or limitation of certain types of damages. If any disclaimer or limitation set forth in this agreement violates the law of the state applicable to this agreement, the agreement shall be interpreted to make the maximum disclaimer or limitation permitted by the applicable state law. The invalidity or unenforceability of any provision of this agreement shall not void the remaining provisions. 1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the trademark owner, any agent or employee of the Foundation, anyone providing copies of Project Gutenberg™ electronic works in accordance with this agreement, and any volunteers associated with the production, promotion and distribution of Project Gutenberg™ electronic works, harmless from all liability, costs and expenses, including legal fees, that arise directly or indirectly from any of the following which you do or cause to occur: (a) distribution of this or any Project Gutenberg™ work, (b) alteration, modification, or additions or deletions to any Project Gutenberg™ work, and (c) any Defect you cause. Section 2. Information about the Mission of Project Gutenberg™ Project Gutenberg™ is synonymous with the free distribution of electronic works in formats readable by the widest variety of computers including obsolete, old, middle-aged and new computers. It exists because of the efforts of hundreds of volunteers and donations from people in all walks of life. Volunteers and financial support to provide volunteers with the assistance they need are critical to reaching Project Gutenberg™’s goals and ensuring that the Project Gutenberg™ collection will
- 72. remain freely availablefor generations to come. In 2001, the Project Gutenberg Literary Archive Foundation was created to provide a secure and permanent future for Project Gutenberg™ and future generations. To learn more about the Project Gutenberg Literary Archive Foundation and how your efforts and donations can help, see Sections 3 and 4 and the Foundation information page at www.gutenberg.org. Section 3. Information about the Project Gutenberg Literary Archive Foundation The Project Gutenberg Literary Archive Foundation is a non-profit 501(c)(3) educational corporation organized under the laws of the state of Mississippi and granted tax exempt status by the Internal Revenue Service. The Foundation’s EIN or federal tax identification number is 64-6221541. Contributions to the Project Gutenberg Literary Archive Foundation are tax deductible to the full extent permitted by U.S. federal laws and your state’s laws. The Foundation’s business office is located at 809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887. Email contact links and up to date contact information can be found at the Foundation’s website and official page at www.gutenberg.org/contact Section 4. Information about Donations to the Project Gutenberg Literary Archive Foundation Project Gutenberg™ depends upon and cannot survive without widespread public support and donations to carry out its mission of increasing the number of public domain and licensed works that can be freely distributed in machine-readable form accessible by the widest array of equipment including outdated equipment. Many
- 73. small donations ($1to $5,000) are particularly important to maintaining tax exempt status with the IRS. The Foundation is committed to complying with the laws regulating charities and charitable donations in all 50 states of the United States. Compliance requirements are not uniform and it takes a considerable effort, much paperwork and many fees to meet and keep up with these requirements. We do not solicit donations in locations where we have not received written confirmation of compliance. To SEND DONATIONS or determine the status of compliance for any particular state visit www.gutenberg.org/donate. While we cannot and do not solicit contributions from states where we have not met the solicitation requirements, we know of no prohibition against accepting unsolicited donations from donors in such states who approach us with offers to donate. International donations are gratefully accepted, but we cannot make any statements concerning tax treatment of donations received from outside the United States. U.S. laws alone swamp our small staff. Please check the Project Gutenberg web pages for current donation methods and addresses. Donations are accepted in a number of other ways including checks, online payments and credit card donations. To donate, please visit: www.gutenberg.org/donate. Section 5. General Information About Project Gutenberg™ electronic works Professor Michael S. Hart was the originator of the Project Gutenberg™ concept of a library of electronic works that could be freely shared with anyone. For forty years, he produced and distributed Project Gutenberg™ eBooks with only a loose network of volunteer support.
- 74. Project Gutenberg™ eBooksare often created from several printed editions, all of which are confirmed as not protected by copyright in the U.S. unless a copyright notice is included. Thus, we do not necessarily keep eBooks in compliance with any particular paper edition. Most people start at our website which has the main PG search facility: www.gutenberg.org. This website includes information about Project Gutenberg™, including how to make donations to the Project Gutenberg Literary Archive Foundation, how to help produce our new eBooks, and how to subscribe to our email newsletter to hear about new eBooks.
- 75. Welcome to ourwebsite – the perfect destination for book lovers and knowledge seekers. We believe that every book holds a new world, offering opportunities for learning, discovery, and personal growth. That’s why we are dedicated to bringing you a diverse collection of books, ranging from classic literature and specialized publications to self-development guides and children's books. More than just a book-buying platform, we strive to be a bridge connecting you with timeless cultural and intellectual values. With an elegant, user-friendly interface and a smart search system, you can quickly find the books that best suit your interests. Additionally, our special promotions and home delivery services help you save time and fully enjoy the joy of reading. Join us on a journey of knowledge exploration, passion nurturing, and personal growth every day! ebookbell.com